"Think to drink" has become a new mantra for exercisers who recognize that proper hydration is essential to athletic performance and overall health.

If you wait until your brain signals "thirst," you're already dehydrated, note sports medicine experts who advise active people to make a conscious effort to drink water regularly.

"During exercise, humans typically drink insufficient volumes of fluid to offset sweat losses," notes the American College of Sports Medicine (ACSM) in its position statement on exercise and fluid replacement.

"At best, voluntary drinking only replaces about two-thirds of the body water lost as sweat. It is common for individuals to dehydrate by 2 percent to 6 percent of their body weight during exercise in the heat despite the availability of adequate amounts of fluid."

In the extreme, dehydration can lead to death. Yet even small losses can be harmful. Losing as little as 1 percent of body weight "can increase cardiovascular strain," the position statement notes, "and increase the probability for impairing exercise performance and developing heat injury."

Get in the habit of drinking fluids before, during and after any form of activity, urge the ACSM experts, who advise exercisers to:

* Drink 500 ml (about 17 ounces) of fluid two hours before exercise to allow time for both adequate hydration and excretion of excess water.

* Start drinking early and continue drinking at regular intervals during exercise. Try to drink five to 12 ounces every 15 to 20 minutes during activity.

* Rehydrate after exercise to ensure that your body is adequately prepared for your next workout.

While plain water is fine for people who exercise one hour or less, the guidelines advise athletes involved in intense activity lasting longer than an hour to drink a beverage containing some sodium plus 4 percent to 8 percent carbohydrate in the form of sugars (glucose or sucrose) or starch (maltodextrin).

Most commercially available sports drinks supply these nutrients, which optimize the body's ability to absorb water and replace depleted energy stores. But homemade sports drinks can work just as well, if they're within the salt and sugar guidelines. Try this one from "The New York City Marathon Cookbook," by Nancy Clarke (Rutledge Hill Press, 1994):

In a glass, dissolve one tablespoon of sugar and a pinch of salt in a little bit of hot water. Add one tablespoon of orange juice or two tablespoons of lemon juice and 7 1/2 ounces of ice water.

Since people will typically drink more of a beverage if it's flavored and chilled, a slightly sweetened drink served at a cool temperature is a good choice, particularly for children and older adults who may need extra encouragement to drink as much as they need.

Be sure to have a drink bottle handy whenever you're going to be active, and carry it in a container that makes it as easy as possible to drink.