To keep the body humming, drink at least eight glasses of water during the course of a day—more if it's hot or dry, or when you are working out. In fact, it is especially important to drink before you begin exercising, to drink as you go, and to rehydrate after your workout. If you wait until you are thirsty to drink, you have waited too long.
Most experts recommend drinking about 16 ounces of water one to two hours before exercising. One hour is the commonly accepted standard, but if you hydrate two hours before working out, it will allow you to eliminate any excess fluid before you begin, and you won't have to interrupt your workout to use the restroom. You should drink four ounces of water every 15 to 30 minutes while you workout. The harder you exercise and the hotter it is, the more you'll have to drink during your workout. It's a good idea to keep a water bottle handy so that you can sip as you need to.
If you're new to working out and drinking as much water as you need, you might have to start both slowly. Just as you progress from lighter weights to heavier ones, from less to more time and tension on an aerobic apparatus, and from a short walk or run to a longer one, up your daily water intake. Drink one additional glass each day until eight is your standard—and remember to drink more when you're working hard or in a hot or dry place.
When you get to the point where your workout lasts longer than an hour—especially if you perspire heavily—consider a sports drink. They not only rehydrate your body, they replace electrolytes (sodium and sodium chloride are the most common) and provide carbohydrates to help you re-energize during a long spell of physical exertion. You lose electrolytes through perspiration, and electrolyte-depletion can cause cramping.