Acceptable prayer is…

How lovely is prayer, and how radiant are its works. Prayer is acceptable to God when it goes with good deeds, and it is heard when it rises out of a spirit of forgiveness. Prayer is always answered when it is pure and sincere. Prayer is powerful when it is suffused with God’s vigor.
–Saint Aphrahat the Persian (270-345)

The purpose of…

The purpose of prayer is not to inform our Lord what you desire, for He knows all your needs. It is to render you able and ready to receive the grace which our Lord will freely give you. This grace cannot be experienced until you have been refined and purified by the fire of desire in devout prayer. For although prayer is not the cause for which our Lord gives grace, it is nevertheless the means by which grace, freely given, comes to the soul.
–Walter Hilton (1340-1396)

Make the psalms…

Make the psalms your own. Do not sing them as verses composed by another person. Let them be born in your own prayers. When they come from your lips, understand that they were not merely fulfilled temporarily when they were first written. They are being fulfilled now in your daily life.
–Saint John Cassian (c. 360-435)

Give your intentions…

Give your intentions in prayer to God, Who knows everyone, even before our birth. And do not ask that everything will be according to your will, because a man does not know what is profitable for him. But say to God: Let Thy will be done! For He does everything for our benefit.
— Saint Gennadius of Constantinople (Fifth Century)