Repeat of previous warnings - DRINK LOTS OF WATER.
Drink after you shoot while you're scoring, or watching the loading or unloading table and between times when you're picking up brass. By the time you actually feel thirsty, it's too late.
Some of the first symptoms of dehydration are headache, sick to stomach, dizziness. By then you need to remove yourself from the heat if possible and drink water or Gatorade, etc.
Cool off by removing your hat and putting damp bandana on your head if you don't want to pour water over your head (which is faster, but wetter). Remember you lose lots of body heat through your head. Drink water.
Sponge your face, neck, and wrists with cool bandana also. Drink water.
I have some of those neck coolers that you soak in water that swell up and stay cool of an hour or so, but they can be messy and drippy and don't work as well in high humidity. I sometimes take ice and wrap it in my bandana and place it on my head and neck to cool off when I start getting too hot.
The main thing is to drink lots of water. Don't wait until you've started to get dehydrated, and don't drink Coke and other carbonated beverages thinking that you're hydrating yourself. Actually you're doing the opposite. Drink water or Gatorade.
Have I said Drink Plenty of Water?