Prayer is the activity which is appropriate to the dignity of the spirit. When you are praying, do not fancy that divinity is like some image formed within yourself. Avoid allowing your mind to be impressed with the seal of some particular shape. Draw near the immaterial being and you will attain to understanding. Stand guard over your mind, keeping it free of concepts at the time of prayer so that it will remain in its own deep calm. A man in chains cannot run nor can the mind that is enslaved to passion see the place of spiritual prayer.
"Do not pray for the fulfillment of your wishes, for they may not accord with the will of God. But pray as you have been taught, saying: Thy will be done in me (cf. Luke 22:42). Always entreat Him in this way - that His will be done. For He desires what is good and profitable for you, whereas you do not always ask for this." - Evagrius Ponticus.
Swami Sarvapriyananda speaks on Evagrius's teachings on prayer.