Singer Britney Spears is showing off a slim and trim bikini body, and her trainer, Tony Martinez, is revealing how she lost the weight. The 31-year-old mother of two boys first showed off her new figure on last month’s cover of Shape magazine and flaunted flat, toned abs. She is set to perform in Las Vegas, Nev., at Planet Hollywood for two years and perform 70 shows beginning later this year.

Spears’ weight has been up and down throughout her career since her “Baby One More Time” days back in 1999.

Martinez, who has also trained celebrities such as Pink and Janet Jackson, told E!, “She is a tremendous athlete. She likes to be very strong and aggressive in her workouts. I kept telling her to stop making it look too easy. It was like a competition with us! He has instructed her to eat small meals five times a day, with her favorite meal being chicken salad with balsamic dressing.”

Fourteen years after the start of her career, Spears admits that keeping her body slim and trim is not as easy as when she was a teenager.

"Dieting is tough, because my specialties are spaghetti, brownies and chocolate chip cookies," she admitted to Shape. "I try not to make those too often, or I’ll eat the whole bowl of dough.”

Spears follows the Nutrisystem weight loss program, where meals consist of a shake for breakfast and lunch plus a dinner of protein and vegetables in order to stay healthy. She also does yoga and cardio to stay fit.

“I’m doing 90 minutes of yoga twice a week in addition to a lot of cardio -- hourlong sessions three times a week with my trainer,” Spears said. “The [Vegas] performances won’t be simple -- they’ll be a massive party from start to finish, and, to pull this off, I have to be in top condition and running at full speed."