Pray  first, for purification from the passions;
 and second, for deliverance from ignorance and forgetfulness;
 and third, for deliverance from all temptation and abandonment.
In your prayer seek only righteousness and the kingdom, namely, virtue and knowledge; and all the rest will be added unto you (Mt 6:33).
It is just to pray not only for your own purification, but also to pray for your own kindred, so as to imitate the angelic way.
[...] Whether you pray with brothers or by yourself, struggle to pray not only in the customary way, but also with perception.
Perception in prayer is concentration (sunnoia), with reverence and compunction and distress of soul, as you confess your failures with silent groans.
If the intellect (nous) is still staring around at the time of prayer, it does not yet know how to pray as a monk; it is still a secular, decorating the exterior tabernacle (cf. Mt 23:27).
When you pray, keep powerful guard over your memory: in this way, instead of placing its own passions before you, it will, instead, move you to the knowledge that you stand before God.
For the nous is easily, naturally disarmed and plundered by the memory at the time of prayer.
When you are praying the memory brings you fantasies of either:  ancient issues;  or new worries;  or the face of one who has distressed you.
The demon is very malignant towards any person who prays, and it employs every means to defeat his purpose.
It does not cease  moving thoughts (noemata) of matters through the memory and  stirring up all the passions through the flesh, so as to be able to impede his excellent course and his departure to God.
When, despite all his efforts, the malevolent demon is unable to hinder the prayer of one who is earnest, it lets up for a time and then takes its revenge when he finishes praying. It either:
 enflames him with anger, thus ruining the excellent state that, through prayer, has been welded together in him;
 or it entices him to some irrational pleasure and so commits an outrage on the nous.
Having prayed properly, expect what is improper; and stand courageously to keep guard over your harvest.
Indeed from the beginning you were assigned this: namely, to work and keep guard (Gen. 2:15). So do not leave your work unguarded after your labor, otherwise you do not receive any benefit from praying.
Evagrius Ponticus (345-399): On Prayer, 37-49, translated by Luke Dysinger OSB.