When A-listers like Nicole Richie, Gwyneth Paltrow and Gisele Bundchen need to shed a few extra pounds, they turn to celebrity trainer Tracy Anderson.

"When I first came to a class of yours, I heard I was going to be there for and hour and a half, and I was like, 'Oh, I don't do that,'" Richie, 29, says. "Now I'm in there two hours a day, four days a week. It's so fun and the time flies by. The music is great and I love the cardio."

"The girl can dance! She's a rock star," Anderson raves of working mom Richie, who burns excess calories while dancing to Jennifer Lopez's "On the Floor" (watch above).

"The dancing is so much fun for me," Richie explains of Anderson's routine. "This is the only workout that I've ever done that I've been able to eat whatever I want. I feel so great after I do it."