[This is an excerpt from an article on the subject from the Chabad website]
The day before...
- Hydrate! Most of the unpleasantness associated with a fast does not come from lack of food, but rather, lack of fluid. The solution is to drink as much water as possible before the fast. Although you may feel you’re about to float off, it will be worth it by the time the fast is well underway. Beware of beer or other alcoholic beverages; they will only dehydrate you. Water or diluted orange juices are the safest options.
- Don't over-stuff yourself before the fast. Many people seem to think that eating a lot the day before will compensate for not eating on the fast day. This will actually make you hungrier. Have you ever noticed how much hungrier you are the morning after a large meal...?2 Eat a proper meal that emphasizes carbohydrates, some protein and foods high in oils and fats since they delay the emptying of the stomach, thus prolonging the effects of your pre-fast meal. Consuming carbohydrates (i.e. potatoes, pasta) will be very effective as they bond with water that your body will make use of during the fast.
- Avoid salty or spicy foods. Salt causes a person to feel thirsty despite having a "normal" amount of water, because extra water is required to absorb the extra salt. For this reason you should refrain from processed foods containing lots of salt such as pickles or cold cuts. Most tomato sauces, canned fish and smoked fish should also be avoided.
- Salads and other high fiber foods that are so important in one's normal diet should be de-emphasized for the pre-fast meal since they travel quickly through the digestive system. Fruit, despite its high fiber content, is worthwhile since it carries a lot of water in a "time-release" form.
(from Chabad website article at http://www.chabad.org/holidays/JewishNewYear/template_cdo/aid/1638389/jewish/Tips-for-an-Easier-Fast.htm )
צום קל/ May you have an easy fast!