If you run, don't do distances greater than 10 miles when the temperature is above 83 degrees. Drink plenty of fluids. Force yourself to drink more than you want, particularly during periods of prolonged activity. We are the only animals that will voluntary drink less fluid than we need. Our four-legged friends know better when to take a drink than we do.
Some experts argue that water is the best.
Others recommend drinks that contain a pinch of sugar and salt and some potassium. Gator Aid is a good example. The important point is to frequently replenish your body with fluids while you are exercising. Runners, for instance, should have something to drink perhaps every 2.5 miles. It is also a very good idea to drink at least 12 ounces of fluids 10 minutes or so before you start your activity.