St. Francis de Sales on Fasting:
The third condition necessary for fasting well is to look to God and to do everything to please Him, withdrawing within ourselves in imitation of a great saint, St. Gregory the Great, who withdrew into a secret and out-of-the-way place where he remained for some time without anyone knowing where he was, being content that the Lord and His angels knew it.
This is all that I had to tell you regarding fasting and what must be observed in order to fast well. The first thing is that your fast should be entire and universal; that is, that you should make all the members of your body and the powers of your soul fast: keeping your eyes lowered, or at least lower than ordinarily; keeping better silence, or at least keeping it more punctually than is usual; mortifying the hearing and the tongue so that you will no longer hear or speak of anything vain or useless; the understanding, in order to consider only holy and pious subjects; the memory, in filling it with the remembrance of bitter and sorrowful things and avoiding joyous and gracious thoughts; keeping your will in check and your spirit at the foot of the crucifix with some holy and sorrowful thought. If you do that, your fast will be universal, interior and exterior, for you will mortify both your body and your spirit. The second condition is that you do not observe your fast or perform your works for the eyes of others. And the third is that you do all your actions, and consequently your fasting, to please God alone, to whom be honor and glory forever and ever.