IF YOU'RE NOT A SERIOUS COMPETITOR, the rules of hydration are simple: (1) Drink when you're thirsty; and (2) you'll know you're hydrated when your urine is pale or clear, not dark. But everyone has different sweat rates, so to drink like a pro, you need to do some quick math. Grab a cold one and follow these directions.

Training: Hydrating simply means replacing lost sweat, so start by figuring your personal sweat rate, which can be as much as two liters per hour. Weigh yourself naked before and after one hour of training. (Plan not to urinate during that time.) Your hourly sweat rate is found by subtracting your postworkout weight from your pre-workout weight and then adding any water you drank—15 fluid ounces of water weighs one pound.

Pre-race: Drink about one ounce for every ten pounds of body weight at least four hours before exercise. Two hours prior, your urine should look like lemonade, not apple juice.

During: As a general rule, drink about eight ounces every 15 minutes, but adjust to your own sweat rate. Make sure you're getting about 100 to 200 milligrams of sodium in every eight ounces, which will help you absorb fluid.

Post-race: Aim for 20 to 24 ounces of salty fluid for every pound of weight lost during exercise, plus some recovery fuel—about half a gram of carbs for every pound of body weight and up to 20 grams of protein.