Biblical references...
Exodus 9:29... Moses said to him, “As soon as I have gone out of the city, I will stretch out my hands to the Lord.  The thunder will cease, and there will be no more hail, so that you may know that the earth is the Lord’s.
Joshua 7:6...  Then Joshua tore his clothes and fell to the earth on his face before the ark of the Lord until the evening, he and the elders of Israel.  And they put dust on their heads.
1st Samuel 1:26... And she said, “Oh, my lord!  As you live, my lord, I am the woman who was standing here in your presence, praying to the Lord.
1st Samuel 12:23... Moreover, as for me, far be it from me that I should sin against the Lord by ceasing to pray for you, and I will instruct you in the good and the right way.
1st Samuel 16:7... But the Lord said to Samuel, “Do not look on his appearance or on the height of his stature, because I have rejected him.  For the Lord sees not as man sees: man looks on the outward appearance, but the Lord looks on the heart.”
2nd Samuel 7:18... Then King David went in and sat before the Lord and said, “Who am I, O Lord God, and what is my house, that you have brought me thus far?
1st Kings 8:54... Now as Solomon finished offering all this prayer and plea to the Lord, he arose from before the altar of the Lord, where he had knelt with hands outstretched toward heaven.
1st Kings 18:42... So Ahab went up to eat and to drink.  And Elijah went up to the top of Mount Carmel.  And he bowed himself down on the earth and put his face between his knees.
1st Chronicles 4:10... Jabez called upon the God of Israel, saying, “Oh that you would bless me and enlarge my border, and that your hand might be with me, and that you would keep me from harm so that it might not bring me pain!”  And God granted what he asked.
2nd Chronicles 6:13... Solomon had made a bronze platform five cubits long, five cubits wide, and three cubits high, and had set it in the court, and he stood on it. Then he knelt on his knees in the presence of all the assembly of Israel, and spread out his hands toward heaven, 
Ezra 9:5...  And at the evening sacrifice I rose from my fasting, with my garment and my cloak torn, and fell upon my knees and spread out my hands to the Lord my God,
Nehemiah 8:6... And Ezra blessed the Lord, the great God, and all the people answered, “Amen, Amen,” lifting up their hands.  And they bowed their heads and worshiped the Lord with their faces to the ground.
Psalms 63:4...  So I will bless you as long as I live; in your name I will lift up my hands.
Psalms 95:6...  Oh come, let us worship and bow down; let us kneel before the Lord, our Maker!
Psalms 100... (A Psalm for giving thanks).  Make a joyful noise to the Lord, all the earth!  Serve the Lord with gladness! Come into his presence with singing!  Know that the Lord, he is God!  It is he who made us, and we are his; we are his people, and the sheep of his pasture.  Enter his gates with thanksgiving, and his courts with praise.  Give thanks to him; bless his name!  For the Lord is good; his steadfast love endures forever, and his faithfulness to all generations.
Isaiah 1:15... When you spread out your hands, I will hide my eyes from you; even though you make many prayers, I will not listen; your hands are full of blood.
Daniel 6:10...  When Daniel knew that the document had been signed, he went to his house where he had windows in his upper chamber open toward Jerusalem.  He got down on his knees three times a day and prayed and gave thanks before his God, as he had done previously.
Matthew 6:5...  “And when you pray, you must not be like the hypocrites.  For they love  to stand and pray in the synagogues and at the street corners, that they may be seen by others.  Truly, I say to you, they have received their reward. 
Matthew 6:6...  But when you pray, go into your room and shut the door and pray to your Father who is in secret.  And your Father who sees in secret will reward you.
Matthew 6:9-13 (King James Version)... After this manner therefore pray ye:  Our Father which art in heaven, Hallowed be thy name.  Thy kingdom come.  Thy will be done in earth, as it is in heaven.  Give us this day our daily bread.  And forgive us our debts, as we forgive our debtors.  And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen. 
Matthew 6:14-15...  For if you forgive others their trespasses, your heavenly Father will also forgive you, but if you do not forgive others their trespasses, neither will your Father forgive your trespasses.
Matthew 11:25-27... At that time Jesus declared, “I thank you, Father, Lord of heaven and earth, that you have hidden these things from the wise and understanding and revealed them to little children; yes, Father, for such was your gracious will.  All things have been handed over to me by my Father, and no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him.
Matthew 14:23... And after he had dismissed the crowds, he went up on the mountain by himself to pray. When evening came, He was there alone, 
Matthew 19:13... Then children were brought to him that he might lay his hands on them and pray.  The disciples rebuked the people,
Matthew 26:26-27... Now as they were eating, Jesus took bread, and after blessing it broke it and gave it to the disciples, and said, “Take, eat; this is my body.”  And he took a cup, and when he had given thanks he gave it to them, saying, “Drink of it, all of you,
Matthew 26:36... Then Jesus went with them to a place called Gethsemane, and he said to his disciples, “Sit here, while I go over there and pray.” 
Matthew 26:39... And going a little farther he fell on his face and prayed, saying, “My Father, if it be possible, let this cup pass from me; nevertheless, not as I will, but as you will.”
Matthew 26:42... Again, for the second time, he went away and prayed, “My Father, if this cannot pass unless I drink it, your will be done.”
Matthew 26:44... So, leaving them again, he went away and prayed for the third time, saying the same words again.
Mark 1:35...  And rising very early in the morning, while it was still dark, he departed and went out to a desolate place, and there he prayed.
Mark 11:25...  And whenever you stand praying, forgive, if you have anything against anyone, so that your Father also who is in heaven may forgive you your trespasses."
Luke 3:21... Now when all the people were baptized, and when Jesus also had been baptized and was praying, the heavens were opened, 
Luke 5:16... But he would withdraw to desolate places and pray.
Luke 6:12... In these days he went out to the mountain to pray, and all night he continued in prayer to God. 
Luke 9:18... Now it happened that as he was praying alone, the disciples were with him.  And he asked them, “Who do the crowds say that I am?”
Luke 9:28-29... Now about eight days after these sayings he took with him Peter and John and James and went up on the mountain to pray.  And as he was praying, the appearance of his face was altered, and his clothing became dazzling white. 
Luke 11:1-4... Now Jesus was praying in a certain place, and when he finished, one of his disciples said to him, “Lord, teach us to pray, as John taught his disciples.”  And he said to them, “When you pray, say:  “Father, hallowed be your name.  Your kingdom come.  Give us each day our daily bread, and forgive us our sins, for we ourselves forgive everyone who is indebted to us.  And lead us not into temptation.”
Luke 18:10-14... “Two men went up into the temple to pray, one a Pharisee and the other a tax collector.  The Pharisee, standing by himself, prayed thus: ‘God, I thank you that I am not like other men, extortioners, unjust, adulterers, or even like this tax collector.  I fast twice a week; I give tithes of all that I get.’  But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, ‘God, be merciful to me, a sinner!’  I tell you, this man went down to his house justified, rather than the other.  For everyone who exalts himself will be humbled, but the one who humbles himself will be exalted.”
Luke 22:32... but I have prayed for you that your faith may not fail.  And when you have turned again, strengthen your brothers.”
Luke 23:34... And Jesus said, “Father, forgive them, for they know not what they do.”  And they cast lots to divide his garments. 
Luke 24:30... And when he had said this, he showed them his hands and his feet.
John 6:11... Jesus then took the loaves, and when he had given thanks, he distributed them to those who were seated.  So also the fish, as much as they wanted.
John 12:27-28... “Now is my soul troubled.  And what shall I say?  ‘Father, save me from this hour’? But for this purpose I have come to this hour.  Father, glorify your name.”  Then a voice came from heaven:  “I have glorified it, and I will glorify it again.”
John 12:27-28 (The Passion Translation)...  “Even though I am torn within, and my soul is in turmoil, I will not ask the Father to rescue me from this hour of trial. For I have come to fulfill my purpose —to offer myself to God. So, Father, bring glory to your name!”  Then suddenly a booming voice was heard from the sky, “I have glorified my name! And I will glorify it through you again!”
John 17:1... When Jesus had spoken these words, he lifted up his eyes to heaven, and said, “Father, the hour has come; glorify your Son that the Son may glorify you,
Romans 8:26... Likewise the Spirit helps us in our weakness.  For we do not know what to pray for as we ought, but the Spirit himself intercedes for us with groanings too deep for words.
Ephesians 5:20... giving thanks always and for everything to God the Father in the name of our Lord Jesus Christ.
Ephesians 6:18... praying at all times in the Spirit, with all prayer and supplication.  To that end, keep alert with all perseverance, making supplication for all the saints,
1st Thessalonians 5:16-18... Rejoice always, pray without ceasing, give thanks in all circumstances; for this is the will of God in Christ Jesus for you. 
1st Timothy 4:4-5... For everything created by God is good, and nothing is to be rejected if it is received with thanksgiving, for it is made holy by the word of God and prayer.
James 4:17... So whoever knows the right thing to do and fails to do it, for him it is sin.

Discussion Thoughts...
(1) A book on Basic Theology by Charles C. Ryrie explains that Romans 8:26 and Ephesians 6:18 tells us that "we do not know what to pray for as we ought" and that the Spirit helps us by guiding and directing our prayers!
(2) Peloubet's Bible Dictionary provides several "logical" statements and scriptures...
      (a) "Prayer implies a personal God, who is our Father." 
      (b) "Many times the exact thing we ask for, would be the worst thing for us, and what we
             really do not want.  We ignorant of ourselves, beg often our own harm... therefore our
             heavenly Father gives us not merely a substitute for what we asked, but what we really
             wanted, not what we thought we wanted."
       (c) Prayer positions:
             Standing - 1st Samuel 1:28 - Matthew 6:5 - Mark 11:25 - Luke 18:11
             Sitting - 2nd Samuel 7:18
             Kneeling - 1st Kings 8:54 - 2nd Chronicles 6:13 - Ezra 9:5 - Psalms 95:6 - Daniel 6:10
             Prostate with face to the ground - Joshua 7:6 - 1st Kings 18:42 - Nehemiah 8:6
             Hands lifted with upturned hands - Psalms 63:4 - Exodus 9:29 - Isaiah 1:15
(3) Prayers in the Bible... The Readers Digest Family Guide to the Bible lists chapter and verse where you can read about 341 mentions of prayer... The Bible Gateway says prayer is mentioned 375 times in the bible... The NIV Exhaustive Concordance says prayer is mentioned 375 times in the bible... a source on the web says prayer is mentioned over 650 times in the bible.
(4) The prayers of Jesus... Halley"s Bible Handbook contains an interesting discussion of the prayers of Jesus... Jesus, although he was the son of God, during his time on earth in the flesh, he seems to have felt himself dependent on a power higher than himself; and he prayed a great deal... listed here are some of his prayers... notice that in every recorded prayer, Jesus addressed God as "Father":
             At his baptism (Luke 3:21)
             In a solitary place (Mark 1:35)
             In the wilderness (Luke 5:16)
             All night, before choosing the 12 (Luke 6:12)
             Before his invitation, "Come unto Me (Matthew 11:25-27)
             At the feeding of the 5,000 (John 6:11)
             After the feeding of the 5,000 (Matthew 14:23)
             When He gave the Lord's Prayer (Luke 11:1-4)
             At Caesarea-Philippe (Luke 9:18)  
             Before His Transfiguration (Luke 9:28-29)
             For Little Children (Matthew 19:13)
             Before the raising of Lazarus (John 11:41-42)
             In the Temple (John 12:27-28)... see 4a below
             At the Supper (Matthew 26:26-27)
             For Peter (Luke 22:32)
             For the Disciples (John 17)
             In Gethsemarie (Matthew 26:36, 39, 42, 44)
             On the Cross (Luke 23:34)
             At Emmaus (Luke 24:30)
(4a) When reading John 12:27-28 the description of God speaking where He said “I have glorified it, and I will glorify it again.” seemed strange!  In the scripture list above there is also the text of John 12:27-28 from The Passion Translation which has the text where God is speaking a bit different... “I have glorified my name! And I will glorify it through you again!”... to me this translation seems to make a lot more sense... "personal comment... I am not a proponent of The Passion Translation... but in this instance it seems to make more sense!"
(5) The Lords Prayer (certainly the most popular prayer of any prayer)... at last count there are 72 current bible versions and thus 72 versions of The Lords Prayer... listed above (Matthew 6:9-13) is the King James Version... which seems to be the most popular version for most churches (but many churches seem to make minor changes in the text)!
Halley's Bible Handbook says the following about The Lords Prayer... "we doubt that Jesus intended this as a form to be repeated in unison in religious services.  We rather think that the prayer was meant to be sort of a norm, to guide us in our approach to God."
The two verses following The Lords Prayer (Matthew 6:14-15) are verses that are probably not familiar to many people... the scriptures are a warning from Jesus that we need to be aware of those scriptures!
(6) Other famous prayers...
The Prayer of Jabez (1 Chronicles 4:10) --- This has always been one of my favorite prayers... it is a very simple prayer with a very specific request which God granted!  Sometime a very simple prayer can change the course of our life!
The Prayer of the Publican (Luke 18:13) --- The humble prayer of the tax collector in Luke 18 is a touching reminder of the power of genuine repentance and humility.  This short, simple yet powerful prayer, teaches about the mercy God extends to those who approach Him with humility.
(7) Pray in secret!  Prayer is the expression of ourselves to God... it is a private matter between ourselves and God... not something to talk about.  Follow Jesus example... Jesus and others frequently prayed in secret and often in private places (Mark 1:35 - Matthew 6:6 - Matthew 14:23 - Matthew 26:36 - Matthew 26:42 - Matthew 26:44 - Luke 5:16 - Luke 6:12 - Luke 9:18 - 1st Kings 18:42).  This is not saying pray only in private... we should never be ashamed to pray in public... but, Jesus examples of praying in private is a wonderful example to follow!
(8) How often should we pray? (1st Thessalonians 5:16-18) tells us to pray without ceasing! (Ephesians 6:18) tells us to pray in the Spirit on all occasions with all kinds of prayers and requests.  There are references in the bible to pray 3 times a day... other places the bible says to pray 7 times a day... and as above it says to pray without ceasing! 
In other words, there does not seem to be a fixed number of times you should pray in a day, and that the Bible does not establishes a minimum or maximum number of prayers that should be said daily.  What the bible does teach is that we should be constantly keeping God in our thoughts, whether by voice, by attitude, or with silent prayers!
(9) Is it a sin to not pray?  (James 4:17) provides a very simple straightforward answer!  The bible clearly tells us to pray... if we do not pray, James 4:17 tells us that is a sin!  There are two kinds of sin... sins of commission and sins of omission!  Not praying is a sin of omission!

My Beliefs... A difficult study... sometimes difficult to arrive at definitive conclusions!!!  Different scriptures point out many different types of prayers.  The bible verse John 4:17 is clear that not praying is a sin!  See the discussion above on how often we should pray... certainly the most common prayer is the Lord's Prayer.  Numerous scripture tell us that our private prayers should be in secret (like most of the time when Jesus prayed)!  The bible is also moot on how to pray... there is no secret formula to prayer... you can bow, kneel, stand, or walk... God will hear you pray... in John 17:1 Jesus looked toward heaven when he prayed... you can pray silently or out loud... praying silently sometimes allows more focus on God... some people say it is easier to hear God if they are not doing all the talking (Pray and silently wait for Gods answer)! 

Other studies... this study is a general study on prayer... see a related specific study on pray before eating.