(the below info is cut-and-pasted from a website)
Lots of Water. This may be hard to believe after one is twenty or so hours into a fast, but most healthy adults can survive well over a month without eating. Most of the unpleasantness associated with a fast does not come from lack of food, but rather, from lack of fluid. The solution therefore is to super-hydrate beforehand. “Camel up” before a fast, drinking a great deal the prior afternoon, perhaps two quarts well in advance of your final pre-fast meal. At the time, you may feel you are going to float away. Before the fast is over, you will be glad you did it. Diluted orange juice is a good drink, as is water. Beer or other alcoholic beverages will dehydrate you, so try to avoid them.
Meat and Potatoes. Though you should drink a lot before a fast, you do not need to stuff yourself with food. Eat a normal meal but emphasize carbohydrates like potato or noodle dishes, not proteins or fats. Carbohydrates bond with water which your body can “drink” when it needs to during your fast. Proteins do not. Most of the dramatic but limited weight loss that people on high protein diets experience is lost water that protein molecules cannot hold onto or bring into your system, water that you want around during a fast. I have heard of grandmothers in Europe who fed their families immense starch meals for the better part of the week before a fast and then, at the final meal, encouraged everyone to eat heavy meat dishes. The carbohydrates would provide the necessary water reservoir. The last-minute meat meal would give the comfort of a full stomach for a number of hours. What people who still eat this way before a fast have to consider is whether they really want to take on all those calories. This kind of pre-fast diet might have been suitable for a culture in which meat was a rarity and people were close to involuntary fasting through much of the year. It is not clear that it makes sense today. “Fast food” does not need to be hopelessly bland, but go easy on the salt, which may make you thirsty. Season with non-irritating spices and herbs.
Can the Caffeine. The nausea and headaches that many people report during a fast have nothing to do either with food or fluid. They are usually the result of caffeine withdrawal. If you are a heavy coffee or cola drinker, start tapering off a week or so before the fast. Unless you drink a great deal of caffeine, one cup less a day, with the day before the fast being caffeine free, will usually do it. Using decaffeinated coffee during this period may help you fool your system. Caffeine withdrawal symptoms are less of a problem when you are eating and drinking than when fasting.
(I’ve sometimes taken asprin before fasts as a pre-emtive measure. It seems to help somewhat, but this does not mean it will work for you or that it’s smart.)