Good advice from Megan. You might try fasting one meal a week, then 2 meals a week, then a 24 hour fast, gradually getting to 24-36 hours every week as a practice. See Bragg's book, "The Miracle of Fasting," the oldest used copy you can find online.
When you're able to do 36 hours with no symtoms then your ready for a 3 day fast. He has a program of gradual increments. Of course the diet may clean you up pretty good and if you eat clean for about 4 weeks you may have no headaches. However, some people seem to get them.
Mike