Not to accept some temptation through one sense or another is easily managed; but it is very difficult to control the imagination and memory of it, once it is accepted. For example, to see or not to see some face, or to look at it with passion or without passion, is not very difficult and does not need much effort; but after you have seen it and looked at it with passion, to banish from your memory the image of this face is already not easy, but demands much effort and no small inner struggle. And the enemy can play with your soul as with a ball, tossing your attention from one memory to another, stirring up desires and passions beneath them, and so keeping you always in a passionate state. Therefore I say to you: stay awake and, above all, watch imagination and memory. - Lorenzo Scupoli (Unseen Warfare: Chapter 26)
In diligent exercise of mystical contemplation, leave behind the senses and the operations of the intellect, and all things sensible and intellectual, and all things in the world of being and non-being, that you may arise by unknowing towards the union, as far as is attainable, with Him who transcends all being and all knowledge. For by the unceasing and absolute renunciation of yourself and of all things you may be borne on high, through pure and entire self-abnegation, into the superessential Radiance of the Divine Darkness. - Dionysius the Areopagite in Mystical Theology, Chapter 1
The Apostle has told us to pray uninterruptedly, without anger or passionate thoughts. And this is excellent advice, for every thought which takes the mind away from God is not merely from the devil but is the devil himself." - St. John Chrysostom.
Try to make your intellect deaf and dumb during prayer, you will then be able to pray. - Evagrios the Solitary, "On Prayer," in the Philokalia
Do not admit any sensory phantasies during prayer, lest you become subject to derangement. - St. John Climacus, "The Ladder of Divine Ascent"
If the practice of prayer is to proceed successfully, it is always essential at the outset to lay everything else aside, so that the heart is completely free of distraction. Nothing should obtrude on the mind: neither face, nor activity, nor object. At such a time all is to be driven out." - Bishop Theophan the Recluse
If, then, you wish to behold and commune with Him who is beyond sense-perception and beyond concept, you must free yourself from every impassioned thought. - Evagrios the Solitary, "On Prayer," in the Philokalia