The desire of your heart is itself your prayer. And if the desire is constant, so is your prayer. Not for nothing did the apostle tell us to pray without ceasing. But did he mean that we were to be perpetually on our knees, lying prostrate, or raising our hands? Is this what is meant by praying without ceasing? Even if we admit that we pray in this fashion, I do not believe that we can do so all the time.
Yet there is another, interior kind of prayer without ceasing, namely the desire of the heart. Whatever else you may be doing, if you but fix your desire on God's sabbath rest, your prayer will be ceaseless. Therefore, if you wish to pray without ceasing, do not cease to desire. The constancy of your desire will itself be the ceaseless voice of your prayer. And that voice of your prayer will be silent only when your love ceases. For who are silent if not those of whom it is said: Because evil has abounded, the love of many will grow cold?
The chilling of love means that the heart is silent. If your love is without ceasing, you are always crying out; if you are always crying out, you are always desiring; and if you desire, you are calling to mind your eternal rest in the Lord.
Augustine of Hippo (AD 354-430), Expositions of the Psalms 37, 13-14: CCL 38, 391-392