Jonathan Edwards provided some very good instruction to preachers. Among that advice was a reminder not to neglect is secret fasting.
One thing more I would mention concerning fasting and prayer, wherein I think there has been a neglect in ministers; and that is, that although they recommend and much insist on the duty of secret prayer, in their preaching; so little is said about secret fasting. It is a duty recommended by our Saviour to his followers, just in the like manner as secret prayer is; as may be seen by comparing the 5th and 6th vss. of the 6th chapter of Matthew with vss. 16-18. Though I don't suppose that secret fasting is to be practiced in a stated manner and steady course as secret prayer, yet it seems to me 'tis a duty that all professing Christians should practice, and frequently practice. ~ Jonathan Edwards