“Pray until you pray!” That is Puritan advice. It does not simply mean that persistence should mark much of our praying—though admittedly that is a point the Scripture repeatedly make.
What they meant is that Christians should pray long enough and honestly enough, at a single sessions, to get past the feeling of formalism and unreality that attends not a little praying. We are especially prone to such feelings when we pray for only a few minutes, rushing to be done with a mere duty. To enter the spirit of prayer, we must stick to it for a while. If we “pray until we pray,” eventually we come to delight in God’s presence, to rest in his love, to cherish his will.
The words of Packer in this regard are worth pondering: I start with the truism that each Christian’s prayer life, like every good marriage, has in it common factors about which one can generalize and also uniquenesses which not other Christian’s pray life will quite match. You are you, and I am I, and we must each find our own way with God, and there is no recipe for prayer that can work for us like a handyman’s do-it-yourself manual or cookery book, where the claim is that if you follow th instruction you can’t go wrong. Praying is not like carpentry or cookery; it is the active exercise of a personal relationship, a kind of friendship, with the living God and his Son Jesus Christ, and the way it goes is more under divine control that under ours. Books on praying, like marriage manuals, are not to be treated with slavish superstition, as if perfection of technique is the answer to all difficulties; their purpose, rather, is to suggest things to try. But as in other close relationships, so in prayer: you have to find out by trial and error what is right for you, and you learn to pray by praying. Some of us talk more, others less; some some are constantly vocal, others cultivate silence before God as their way of adoration. . . yet we may all be praying as God means us to do. The only rules are, stay within biblical guidelines and within those guidelines, as John Chapman puts it, “pray as you can and don’t try to pray as you can’t”.
–D. A. Carson, A Call to Spiritual Reformation