Heat exhaustion and its more serious counterpart heat stroke are not to be trifled with. One way to prevent both is to avoid becoming dehydrated. Athletes and avid exercisers are at risk of dehydration if they sweat profusely and don't replenish themselves while working out.
Symptoms of dehydration include feeling dizzy, lightheaded, dry mouth or throat, stomach cramps, and not urinating or producing dark-colored urine. If you notice these symptoms, hydrate yourself immediately with water. To prevent dehydration in the first place, be sure to drink water throughout your day. If you're not sure how much water you should consume daily, then check out Fit's calculator to find out. If you plan on exercising, drink eight ounces of water 20-30 minutes beforehand. Continue to drink four to eight ounces every 10 to 15 minutes of exercise, and drink at least eight ounces after your workout is complete.
Do know that you can overhydrate and experience symptoms of hypnotremia, a condition where the sodium in your blood is diluted by excess water. In really hot, sweaty conditions it is beneficial to drink a sports drink fortified with sodium and electrolytes.