17 Let your companions be women pale and thin with fasting, and approved by their years and conduct; such as daily sing in their hearts: “Tell me where you feed your flock, where you make it to rest at noon,” and say, with true earnestness, “I have a desire to depart and to be with Christ.” Be subject to your parents, imitating the example of your spouse. Rarely go abroad, and if you wish to seek the aid of the martyrs seek it in your own chamber. For you will never need a pretext for going out if you always go out when there is need.
Take food in moderation, and never overload your stomach. For many women, while temperate as regards wine, are intemperate in the use of food. When you rise at night to pray, let your breath be that of an empty and not that of an overfull stomach. Read often, learn all that you can. Let sleep overcome you, the roll still in your hands; when your head falls, let it be on the sacred page. Let your fasts be of daily occurrence and your refreshment such as avoids satiety. It is idle to carry an empty stomach if, in two or three days' time, the fast is to be made up for by repletion.
When cloyed the mind immediately grows sluggish, and when the ground is watered it puts forth the thorns of lust. If ever you feel the outward man sighing for the flower of youth, and if, as you lie on your couch after a meal, you are excited by the alluring train of sensual desires; then seize the shield of faith, for it alone can quench the fiery darts of the devil. “They are all adulterers,” says the prophet; “they have made ready their heart like an oven.” But do you keep close to the footsteps of Christ, and, intent upon His words, say: “Did not our heart burn within us by the way while Jesus opened to us the Scriptures?” and again: “Your word is tried to the uttermost, and your servant loves it.” It is hard for the human soul to avoid loving something, and our mind must of necessity give way to affection of one kind or another.
The love of the flesh is overcome by the love of the spirit. Desire is quenched by desire. What is taken from the one increases the other. Therefore, as you lie on your couch, say again and again: “By night have I sought Him whom my soul loves.” “Mortify, therefore,” says the apostle, “your members which are upon the earth.” Because he himself did so, he could afterwards say with confidence: “I live, yet not I, but Christ, lives in me.” He who mortifies his members, and feels that he is walking in a vain show, is not afraid to say: “I have become like a bottle in the frost. Whatever there was in me of the moisture of lust has been dried out of me.”
And again: “My knees are weak through fasting; I forget to eat my bread. By reason of the voice of my groaning my bones cleave to my skin.”
Source: Letters (New Advent)