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Rx: One hour of circuit training (cardio and resistance) 4x a week, plus at least one day of cardio for 45 to 60 minutes at a high intensity. Take one day off.

With the 30s, you start noticing that weight doesn't come off quite as easily as it used to. This is because after age 20, your basal metabolism drops by 1 to 2 percent every decade, and as lean muscle decreases and body fat increases, you don't need as many calories to sustain yourself. "Exercise is the number one form of preventive medicine," says Jillian Michaels, 32, who was a trainer on the first three seasons of NBC's The Biggest Loser and is the author of Winning by Losing: Drop the Weight, Change Your Life. "You won't see that big a difference between 31 and 39 if you've been living a healthy lifestyle, but if not, you'll see a huge difference in muscle tone, weight, and shape."

In this decade, experts agree, keeping fit means working harder. Jenkins favors circuit training—a series of resistance and cardio exercises done swiftly and back-to-back. But however you do it, Michaels advises strength training each muscle group twice a week with two days of rest between sessions. Don't stick with heavy weights/low reps or low weight/many reps, she says; switch it around to keep your body from getting used to the routine. One day of rest a week is crucial.

After pregnancy a program like Pilates can be invaluable in "pulling everything back in and up," says Brooke Siler, 38, whose re:AB studio in New York City has attracted famous figures like Amber Valletta, Madonna, and Liv Tyler. "I especially like exercises that involve standing, because they teach you to fight what nature wants you to do, which is slump," says Siler, the author of The Pilates Body.

Now is the time to make good fitness habits a part of everyday life. "You always want to be standing instead of sitting, taking stairs instead of elevators," says Siler. "I'm constantly aware of how I sit and stand and walk down the street. I'm forever pulling in and up. These invisible workouts are really important for a woman in her 30s. It's how you start preparing your body for what's to come."
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