Sayings of the Holy Fathers:

Soiled prayer is one thing, its disappearance is another, robbery is another, and blemish another. Prayer is soiled when we stand before God and picture to ourselves irrelevant and inopportune thoughts. Prayer is lost when we are captured by useless cares. Prayer is stolen from us when our thoughts wander before we realize it. Prayer is blemished by any kind of attack or interruption that comes to us at the time of prayer.

St. John Climacus
The Ladder of Divine Ascent, Step28: On Holy and Blessed Prayer, the Mother of Virtues, and on the Attitude of Mind and Body in Prayer

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Some stand before earthly kings without weapons and without armor; but others hold staffs of office, or have shields, or swords.The former are vastly superior to the latter, for they are usually personal relations of the king and members of the royal household. So it is with earthly kings.

St. John Climacus
The Ladder of Divine Ascent, Step 20: On Bodily Vigil, and How to Use It to Attain Spiritual Vigil, and How to Practice It, (Boston: Holy Transfiguration Monastery, 1978)

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"Sometimes prayer seems to flag and we cry, 'Make haste unto me, O God' (Ps. 70:5). But if we do not let go of the hem of His garment, help will come. It is vital to dwell in prayer in order to counteract the persistently destructive influence of the outside world."

Archimandrite Sophrony
(His Life is Mine, Chapter 8; SVS Press pg. 64)