Ye ask, and receive not, because ye ask amiss, that ye may consume it upon your lusts.
But if you pray well, you can always be confident that God will give you either what you asked for, or something better.
Three elements of effective prayer: Humility, Confidence, Perseverance.
Clearly, God enjoys being nagged for things. It pleases Him for us to have a persistent recourse to Him.
5 And He said to them, “Which of you shall have a friend, and go to him at midnight and say to him, ‘Friend, lend me three loaves; 6 for a friend of mine has come to me on his journey, and I have nothing to set before him’; 7 and he will answer from within and say, ‘Do not trouble me; the door is now shut, and my children are with me in bed; I cannot rise and give to you’? 8 I say to you, though he will not rise and give to him because he is his friend, yet because of his persistence he will rise and give him as many as he needs. 9 “So I say to you, ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you. 10 For everyone who asks receives, and he who seeks finds, and to him who knocks it will be opened. 11 If a son asks for bread from any father among you, will he give him a stone? Or if he asks for a fish, will he give him a serpent instead of a fish? 12 Or if he asks for an egg, will he offer him a scorpion? 13 If you then, being evil, know how to give good gifts to your children, how much more will your heavenly Father give the Holy Spirit to those who ask Him!”