One last thing... in addition to eating healthy food's as Chaim promotes, one needs to not miss meals. If you cannot take the time for a full meal, make sure you get something like a bag of almonds and/or some fruit. We fast at certain times of the year, and G-d has taken this into account in how He built us [as well as giving us the intellectual capabilities for understanding in how to "prepare" for a fast in the days before a fast], but we are not designed to fast randomly, or suddenly. -- Some of the worst headaches I have gotten are because I have let myself become too encapsulated by the tasks I undertake during the day and then ignoring my need for food, by skipping breakfast and/or lunch and/or dinner.