Elder Ephraim of Philotheou: Talk only a little, and be watchful over thoughts

 Do not talk a lot. Stay away from back talk, quarreling, loquacity, and everything that issues from a careless tongue. Drive away evil thoughts and filthy fantasies from your mind as soon as they appear. For when the linger inside the mine and heart, they create a grave condition. Whereas when we are careful at the first appearance of the filthy fantasy and the filthy thought that follows, we remain in peace and enjoy the moral gratification of purity.

Therefore, my children, let us pay attention to ourselves every time various bad thoughts enter, because the same approach applies for every bad thought. Whatever kind is may be, when it finds the nous careless, it enters and creates - corresponding to the passion - the aforementioned unhealthy condition.

Therefore, since a monk is fought primarily by thoughts, the thing that saves him is watchfulness! Watchfulness (nepsis) is derived from the word "nēfo," that is, to be careful, vigilant, alert, and on our guard. When we are careful, vigilant, alert, and on our guard, the house of our soul will be well-kept and we shall save our souls for which we struggle our whole life.

+Elder Ephraim of Philotheou, from Counsels from the Holy Mountain, Chapter 14 On Thoughts Fantasies and Distractions, #9

St. Dorotheos of Gaza: On remembering evil

 There are times when, with apparent sincerity, a man asks forgiveness of his brother after some discord between them or for some quarrel which has arisen, and yet after the reconciliation he still remains troubled and has hard thoughts against his brother. He ought not to dwell on such thoughts but to cut them off immediately, for this is remembering evil. It needs much self control not to prolong such thoughts and fall into danger.

+St. Dorotheos of Gaza, from Dorotheos of Gaza: Discourses and Sayings, chapter 8 On Rancor or Animosity.

St. Isaac the Syrian: On the measure of our thoughts

 You may comprehend the measure of your way of life from that which arises in your mind with regard to what is muses on in its thoughts; for which things your nature effortlessly longs, what stirrings are aroused continually, and which are caused by an accidental circumstance; whether your mind has any perception at all of incorporeal thoughts; or whether all its motions are of a material sort; and whether the mind's material quality is something passionate, or only that the thoughts are imprints of the physical aspect of a man's virtuous labor: for the mind involuntarily muses upon the things wherewith it performs the virtues. From these things [last mentioned] the mind, in a wholesome manner, receives the cause of fervor and the gathering of its deliberations, for because of its lack of training the mind, with a good intention, prefers to labor in a corporeal manner, though it does not do so passionately. Observe also whether your mind remains unaffected by hidden confrontations with the imprints of thoughts because of a mightier ardor for the Divine, which is wont to cut off vain recollections.

+St. Isaac the Syrian, from Ascetic Homilies, homily 2.