Today's reading from Luke 11 at Mass contains the familiar "Ask and you shall receive; seek and you shall find; knock and it shall be opened to you." Elsewhere, the Lord advised praying with persistence. Obviously, God doesn't mind our twisting his arm a bit if we feel the need to do so.
When Jesus was going through his agony before his crucifixion, he prayed if the cup could pass him by, yet at the end he said "Your will be done." I think this is the model we should follow. It is consistent with what Jesus teaches in the Our Father he gave us.
By the way, the saints, after having totally abandoned themselves to God, merely ask God to do whatever he wishes with them. This is prayer at the highest level.