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How did Cameron Diaz prep her legs for those super short shorts she donned while filming Bad Teacher over the weekend?

Her longtime trainer, Teddy Bass, tells they mix up weight training and Pilates.

"That seems to be the best combination for women over 30," he explains. "Women need weight -- it's great for building the butt. And elongation and smallness is tapped by doing Pilates."

The two hit the gym "as often as possible," depending on Diaz's work schedule. "You can do a great Pilates session in 45 minutes. If you have an hour-and-a-half, we'll do half weights and half Pilates and flexibility work."

Diaz's gams are worked out by a series of compound exercises, such as a step-up with a reverse lunge, a squat with a curtsy, a side-reaching lunge with a side step, in which you slowly step your feet together. The combination of moves really works the upper thighs, Bass explains. "You can do fewer reps with heavier weights," adds the trainer, who recommends at least 10 to 20-pound weights for lower-body work.

They also do mat Pilates and use the reformer machine. "I keep the workout as high-paced as possible because of time restraints. Generally, it has a cardio element."

You can do the moves without leaving your house.

"Everyone says, 'I don't have money, I can't join a gym.' Most of the work that I do doesn't use a gym," Bass tells Us. "I don't have a reformer machine when I go to set. We don't carry leg presses. You just need 10-pound dumbbells and a four-by-six space."

He continues: "As long as you're doing cardiovascular [work] three to five times a week, leg exercises that build your glutes and working on your elongation with yoga and Pilates, you'll change [your] appearance."