“Julia Roberts and I have been working out together for 20 years,” says Kathy Kaehler, who has also trained Jennifer Aniston, Drew Barrymore, Kim Basinger, Claudia Schiffer and Michelle Pfeiffer. “Our workout is step aerobics—it’s the most amazing way to tone and shape your legs and butt. She travels with my Kathy Kaehler's Fitness System DVD and has me come to her house three days a week. The bottom line is that she is consistent. That’s the key to all the actresses I have trained. They started training and never stopped.”

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To keep Poppy in shape for her hit series, Unforgettable, Joe Dowdell has her perform a series of three mini-circuits with three exercises in each circuit. “For the first two movements, I will choose more traditional types of exercises, like a dumbbell split squat (i.e., A1) and a push-up (i.e., A2) along with a non-traditional movement, such as prowler push, battling ropes or sled pulls with a 30’ rope (i.e., A3),” says Dowdell, founder and co-owner of Peak Performance in NYC and fitness expert on LIVESTRONG.COM’s advisory board. “I give Poppy up to 30 seconds of rest between the A1 and A2 movements and then up to 60 seconds after the third exercise in the circuit. Once we’ve completed the first mini-circuit three times, we’ll move on to the next mini-circuit, which will be comprised of three entirely different movements. I find that this style of training not only allows me to get her stronger and leaner but it also keeps the sessions interesting for her.”

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Trainer Stefan Aschan gets Sports Illustrated centerfold Ana Paula Araujo’s rear into gear with a Swiss ball, which challenges stabilizer muscles by creating instability. Her favorite derriere developer is the “ball bridge.” To try it, lie face up with your back and head on the ball. Lower your butt to the floor for two seconds, then drive it back up in one second. Repeat 20 times. Rest for 30 seconds and then repeat the exercise three more times. In addition to toning your tush, it will also target your hamstrings and core.

After giving birth to her first child, Rules of Engagement star Megyn Price decided to do something for herself: compete in triathlons. In 2011, with two seasons of racing under her belt, she upped the ante and signed up for the Nautica New York City Triathlon, an Olympic-distance race comprosed of a one-mile swim, 25-mile bike and six-mile run. Price trained two hours a day for three months to successfully complete the race—her longest to date. Afterward she told MORE, “The race really seemed out of my reach. But I guess that’s my new mantra as I get older: Find the things that are too big, and then go do them. I feel like when you have a little girl, that’s your responsibility. You have to teach her things like that. You’ve got to go find what scares you, and do it anyway.” Read the full interview here.

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Trainer Michelle Lovitt likes her clients to aim for 20-60 minutes of continuous cardiovascular exercise each session, incorporating intervals in their target heart rate zone. Lauren Graham mixed things up by doing these workouts on her bike, on the treadmill and while hiking, says Lovitt. To find out how to calculate your target heart rate, click here.

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Like Paltrow, Courteney Cox depends on the Tracy Anderson Method to maintain a bikini-ready body. "I stick with the Method because it is a constant challenge," Cox told GOOP, Gwenyth Paltrow’s lifestyle newsletter and website. "I have seen amazing results that I never thought were possible. My butt has lifted, my arms are sculpted and my skin is tight and actually clings to the muscle again."

Kathy Kaehler also helped Cindy Crawford get in shape to tape her “A New Dimension” fitness video. “The one tool that we used that everyone has at their disposal is a deck of playing cards,” says Kaehler. To tighten and tone your tush, try this card-inspired squat exercise: Hold 20 cards in your left hand. Feet should be shoulder width apart and your back straight. Keeping your weight in your heels, squat down (with belly in) and put one card down between your legs. Stand back up. Repeat until all 20 cards are on the floor, then do the exercise in reverse, picking up the cards one by one between squats and putting them in your right hand. 

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Model Emily DiDonato worked with trainer Jay Cardiello to sculpt a body that looked lean and natural but was also functionally strong. “The protocols we used were based on a training system I developed called JCORE, which keeps muscles constantly challenged from all angles and through full ranges of motion,” says Cardiello. “This means that Emily performed multi-joint, body weight exercises for a duration of 30 seconds each, which kept her body in constant movement for 20 minutes working on agility, cardiovascular health, strength and core balance conditioning.” Here’s an example of one move: Standing on your right leg with arms extended laterally, rotate down as fast as possible and touch your left hand to your right foot then stand back up. Repeat for 30 seconds. Switch sides.

Since fitness guru Tracy Anderson has been criticized for being "obsessed" with exercise, it seemed like a match made in exercise heaven when she started working with Madge. In 2009 Madonna told the UK’s Daily Mail, "Tracy Anderson is my savior. After two cesareans, three hernia operations and one riding accident that left me with ten broken bones, she was the only one who could pull my body back together in one piece." But a few months later, after three years together, Madonna and Anderson had a very public split. Madonna reportedly said she grew "tired of the trainer’s personal baggage," when Anderson delayed joining the singer on the European leg of her Sticky & Sweet tour to spend time with her boyfriend. Want more workout tips?