I once heard that in order to prepare for a fast day and keep oneself well hydrated, drink more than usual as much as a week before the fast. I’ve also heard that powerade is good to have before a fast.
On the actual fast, limit physical activity as much as possible (a fast day is not the day to be working out. Not physically or spiritually) and try to keep in a cool area as much as you can. Specifically for Tisha B’av, try to listen to a shiur to keep your mind off the fasting. There are probably things available online.