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20s

Lauren Conrad

The ex-Hills star keeps her fitness routine consistent and watches the Starbucks

HOW SHE WORKS OUT

The fashion designer hits the gym up to four times a week with trainer Jarett Del Bene, who has her doing a workout called body box. "It's high-intensity kickboxing with a lot of body-weight movements, such as squats, lunges and jumping jacks with upper cuts," he says.

HOW SHE EATS

Conrad, 23, tries to stick to around 1,500 calories a day, usually opting for oatmeal with fruit for breakfast, a salad with grilled chicken for lunch and salmon with brown rice for dinner. "She loves Starbucks, but I tell her to lay off," says Del Bene. "When she does want coffee, she drinks it black. That's all you need."

TRAINER TIP

"Stay consistent," says Del Bene, who recommends exercising a minimum of three times a week, 30 minutes at a time. "If you want to burn calories, you have to sweat."

30s

Mel B

The Spice Girl toned up for her Las Vegas stage revue Peepshow

HOW SHE WORKS OUT

The singer, 34, does 30 minutes of cardio a day (usually on a Bowflex machine or treadmill) and up to 600 sit-ups in order to fit into the skimpy lingerie—including a fishnet body suit!—she performs in. "When you hit 30, your metabolism changes dramatically," she recently told PEOPLE. "So I was like, 'Oh, wow, I really need to look after myself now.' "

HOW SHE EATS

Mel relies on the food delivery service Freshology, which serves up meals like egg whites with turkey bacon, salmon salads and grilled chicken with veggies. Once a week, however, she allows herself "my junk food day," which usually involves eating "either an In-N-Out cheeseburger or a traditional English Sunday roast."

HER BEST ADVICE

"Work out with friends!" she says. "It just makes the time fly by quicker."

40s

Valerie Bertinelli

After getting back into a bikini (at 49!), she has maintained her size 6 body

HOW SHE WORKS OUT

After upping her workout routine last spring to get bikini-ready, Bertinelli is now keeping fit with a mix of strength training and cardio four times a week with trainer Christopher Lane. Her next challenge? A marathon for her 50th birthday in April. Having a goal, says Lane, "helps to keep her motivated."

HOW SHE EATS

The Jenny Craig spokeswoman sticks to a 1,700 calorie-a-day diet (up from 1,200) that has her eating both the company's prepackaged food and real food, like sirloin steak and grilled chicken salads. To keep portions in check when eating out, Bertinelli splits an entree with her dining companion.

TRAINER TIP

"I always say that every extra bit counts," says Lane. "Wear a pedometer and find out what your daily step count is, and gradually increase it."

50s

Sharon Stone

Pilates and a lean diet (no bubbly!) keep the star in Basic Instinct shape

HOW SHE WORKS OUT

"We try to stick to a few full-body exercises to really give her a tight core," says Stone's trainer Anne Winfree. "In your 50s it's key to really know how to do abdominal work that supports the spine." That means 60-minute Pilates training sessions three times a week, as well as arm and leg exercises using resistance bands to "target that love-handle area and glutes." Winfree also tries to mix up Stone's routine with hiking and running sessions.

HOW SHE EATS

Stone, 51, eats mostly lean proteins (like chicken and salmon), fruits and whole grains and has a ban on alcohol, caffeine and almost all sugar. "She really wants to be consistent these days," Winfree says.

TRAINER TIP

"You have to have those days when you just let go, and Sharon does," says Winfree. "She likes to have a little bit of chocolate now and again. And a cheeseburger, though very rarely!"