small washcloth,
small, insolated lunchbag with ice. Your favorite, frozen edibles (straberries, blueberries), pack some sliced cucumbers in it.
during change overs, wrap ice in the washclothe and get it to your wrists, back of the neck/shoulders. Keep the ice away from muscle zones or you may cause cramping. Stay out of the AC.
clothing: drifit, cotton if you have to, but don't change clothing ? once. Remember you can do wardrobe change within a reasonable amount of time.
consider what you're drinking. Too much gatorade stuff will give you a sugar crash. For some players, water is enough. For others, they need electrolytes and/or protein drinks during the match. Yummy, fruity flavors!
Know thy own spit. Sounds weird, but an imbalance in you spit is a signal to you.
Take note of where the shade is on the court and where it's migrating. Even if you have 1 foot of shade on your end, got to it whenever you can. If your seat is in the sun, move it to where you want. There's no rule that says you have to sit there.
KNOW the signs of heat exhaustion: cramping; blurred vision; dizziness; nausea, etc. KNOW the medical time out rules.