This post IS MUSIC-RELATED if you gigging in the hot sun.

Self-test for dehydration:

Use thumb and forefinger to pinch the skin on the back of your hand. When released it should return to flat almost instantly. If not, you are dehydrating.

Re-hydration Drink

You can make an inexpensive homemade re-hydration drink. Measure all ingredients precisely. Small variations can make the drink less effective or even harmful.  
Mix:

1 quart (950 ml) water
½ teaspoon (2.5 g) baking soda
½ teaspoon (2.5 g) table salt
3 to 4 tablespoons (45 to 60 g) sugar
If available, add ¼ teaspoon (1.25 g) salt substitute, such as "Lite Salt".

This simple concoction also replaces critical electrolytes lost through perspiration.