Be Happy’s advice above is spot on, as well as eating apples. I was told that apples have alot of water content and that it is healthy to eat a few apples after your main meal, and it also helps the digestion Drink plenty of fluids a few days before the fast. Blublah is also correct. Stay off the caffeine for a few days so that you don’t get headaches from the withdrawal symptoms. Keep cool and stay busy after chazot.
May we all be zocher to have a Yom Tov this Tisha B’Av and see the geulah shelayma!