To be able to offer our prayer with that earnestness and purity with which it ought to be offered, first, all anxiety about carnal things must be entirely got rid of.
Next, we must leave no room for not just the care but even the recollection of any business affairs, and likewise must also lay aside all backbitings, vain and incessant chattering, and buffoonery. Anger above all and disturbing depression must be entirely destroyed, and the deadly taint of carnal lust and covetousness be torn up by the roots.
Then there must be laid the secure foundations of a deep humility, which may be able to support a tower that shall reach the sky; and next the spiritual structure of the virtues must be built up upon them, and the soul kept free from all conversation and from roving thoughts, so that thus it may little by little begin to rise to the contemplation of God and to spiritual insight.
Whatever our mind has been thinking of before the hour of prayer, that thought is sure to occur to us while we are praying—for the mind in prayer is formed by its previous condition. When we are applying ourselves to prayer, the images of the same actions and words and thoughts as in our previous condition will dance before our eyes. They will make us angry or gloomy, or recall our former lust and business, or make us shake with foolish laughter at some silly joke, or smile at some action, or fly back to our previous conversation.
So if we do not want anything to haunt us while we are praying, we should be careful before our prayer to exclude it from the shrine of our heart.
–St. John Cassian, Conferences, 9.3