The Gatorade Sports Science Institute offers these tips that I follow religiously and will paraphrase:

1. Drink 2 servings of sports drinks 2 hours BEFORE strenuous exercise and 2 servings within 30 minutes AFTER.
2. Drink water daily until your, ahem, urine is clear.
3. During a race drink at least 1 cup of water at each station.
4. After 90 minutes of running, drink BOTH water and sports drinks. If you don’t you’ll overheat, cramp, and stall out
5. Drink it, don’t wear it – pouring water on your head won’t cool down your core temperature. Drink it.