Stay hydrated. The easiest way to avoid heat disorders is to keep your body hydrated. This means drinking fluids before, during and after exercise. The body's fluid needs vary with exertion, climate, humidity, terrain, and other factors. The new fluid recommendations for runners say that they should "obey your thirst" and drink when their mouth is dry and they feel the need to drink. In training, drink before workouts and make sure you have access to fluids if exercising longer than 30 minutes. During longer workouts, some of your fluid intake should include a sports drink (like Gatorade) to replace lost salt and other minerals (electrolytes).
Choose clothing carefully. Light-colored, loose-fitting clothing will help your body breathe and cool itself down naturally. Tight clothing restricts that process and dark colors absorb the sun's light and heat. Wear synthetic fabrics (not cotton) because they will wick moisture away from your skin so cooling evaporation can occur.
Don't push it. On a race day (or during any intense workout), take weather conditions into account. Brutal heat and humidity mean you should scale back your performance goals. Don't try to beat the heat.
Be educated. You should be very familiar with the signs of heat problems so you recognize them in yourself or in a running partner. If you feel faint, dizzy, disoriented, have stopped sweating, or your skin is cool and clammy, slow down or stop running. If symptoms continue, sit or lie down in the shade and seek help.
Make a splash. Use water to cool yourself during runs. If you are overheating, splashing water on your head and body will cool you down quickly and have a lasting effect as the water evaporates from your skin.