Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours.
Mark 11:24
Where Jesus Says It
The verses just before this one change how you read it. People quote this verse by itself far more often than they tell the story it comes from.
Jesus has just cursed a fig tree that had leaves and no fruit. Between the cursing and the withering, he goes into the temple and drives out the people selling there, and he says his Father's house was meant to be a house of prayer. The next morning Peter sees the tree and says, "Rabbi, look! The fig tree you cursed has withered!" Jesus answers him with "Have faith in God," then the line about telling a mountain to throw itself into the sea, and then this verse.
Mark puts the temple inside the fig tree story on purpose. Jesus curses the tree, clears the temple, and then the tree is dead. The tree had leaves and no figs. The temple had crowds and coins and traders and no prayer. Mark is telling us they were the same thing, and the tree is what the temple looked like from God's side of it.
So Jesus promises this while he's standing between a fruitless tree and a temple that had stopped being a house of prayer. The people hearing him had just watched him critique religion that produced nothing, and he tells them that faith asks God for things and expects him to move.
He doesn't stop there either. The next sentence is "And when you stand praying, if you hold anything against anyone, forgive them, so that your Father in heaven may forgive you your sins." Jesus makes forgiving people part of the promise itself, rather than a separate teaching he gives somewhere else.
So he never limits what we can ask him for, but he does say we can't be holding a grudge against anybody while we ask.
Ask Big, Hold Nothing
Ask God for the prayer you actually mean, and not the smaller version you've already talked yourself into. Say it plainly and expect him to hear it, because Jesus told you to ask and he wasn't being careless when he said it.
Then forgive whoever came to mind while you were praying. Forgive them today, and not once you feel like forgiving them. You don't have to tell them, and you don't have to feel warm about it, but you do have to stop keeping the account.
I wrote about asking God for big things in More Than You Thought to Ask For, including the hard part, that God sometimes answers no and a no is still an answer. We ask him boldly, and we hold his answer loosely.
I serve at a Vineyard church, and we pray for people to be healed, and we keep praying when nothing visible happens. We're taking Jesus at his word here and letting him be God about the timing.
Prayer
Jesus, you told us to ask you for things, but we often ask you for less than we want. Give us the courage to bring you the prayer we actually mean. Help us to believe you're good and able, and to keep believing while we wait. And where we're holding something against someone, we release it now, before we ask you for anything else. We pray all this in Jesus' name. Amen.