"Peter sent them all out of the room; then he got down on his knees and prayed. Turning toward the dead woman, he said, 'Tabitha, get up.' She opened her eyes, and seeing Peter she sat up." (Acts 9:40)Peter knelt down and prayed for Tabitha (Dorcas) who was dead. God answered Peter's prayer and raised her back to life. A miracle that led many to believe in the Lord! Kneeling down to pray is a posture of humble reverence towards God. We see this several times in Acts. "Then he (Stephen) fell on his knees and cried out, 'Lord, do not hold this sin against them.'" (Acts 7:60a) "When Paul had finished speaking, he knelt down with all of them and prayed." (Acts 20:36) "When it was time to leave, we left and continued on our way. All of them, including wives and children, accompanied us out of the city, and there on the beach we knelt to pray." (Acts 21:5)
More important than our outward posture is our heart's posture/attitude when we pray. We see the attitude contrast between the Pharisee and the tax collector when they went up to the temple to pray in a parable told by Jesus in Luke 18:10-14. "The Pharisee stood by himself and prayed: 'God, I thank you that I am not like other people--robbers, evildoers, adulterers--or even like this tax collector. I fast twice a week and give a tenth of all I get.' But the tax collector stood at a distance. He would not even look up to Heaven, but beat his breast and said, 'God, have mercy on me, a sinner.' I tell you that this man, rather than the other, went home justified before God. For all those who exalt themselves will be humbled, and those who humble themselves will be exalted." Our heart's posture towards God should be: complete surrender, utmost reverence, contrite repentance, utter dependence, deepest gratitude, humble obedience, undivided devotion, highest adoration, and total trust.