Start drinking a lot of water all the way up 4 hours before the game.
*Tonight, eat some whole wheat pasta with some veggies (and a little meat if you want).
If your tryouts are in the late afternoon, eat some fruit and yogurt for breakfast and a small sandwich and fruit for lunch (day of tryouts)
If you game is in the morning, have a banana and a slice of toast for breakfast (day of tryouts).
*Bring a bottle of water and a half bottle of sports drink like gatorade. Drink more water than gatorade during your tryouts.
*Metality--Don't be so nervous. Play your game. Try focusing on having a low turn-over ratio (make good, strong, clean passes).
-Try not to "show off" in order to impress.
It also helps if you verbally communicate with your tryout teammates. Coaches like to see people take leadership positions.
-People with fancy gear and shiny shoes aren't all they are cracked up to be. More than likely, they aren't as good as they think they are.
-Focus on not only what you do with the ball, but what you are doing off of the ball. Make good, supporting runs. Show that you can defend effectively.
*Have solid warmup-- Warm you body up properly then have a good stretch. For tryouts, I usually try to find a group of people warming up that I might get along with. Warm up with them and get some conversation going. Talking will help you get less nervous and more comfortable.
*Wear a good training shirt that stands out a little. This will make it easier for coaches to spot you when you are playing. If you don't have one of these shirts, its ok. Your play will do your talking