Hi, willemina. Neat question, I think.

Dr. John Piper has a GREAT series of sermons that he preached in early 1995 on prayer & fasting. Of those, I would encourage you to look at two of those sermons in particular:

Title: When the Bridegroom Is Taken Away
Subtitle: They Will Fast—with New Wineskins
Date: January 8, 1995
Link: http://www.desiringgod.org/library/sermons/95/010895.html

and

Title: Man Shall Not Live By Bread Alone
Subtitle: What to Eat While Fasting
Date: January 15, 1995
link: http://www.desiringgod.org/library/sermons/95/011595.html

I pray that these will be helpful to you!

In Christ,
Ted