Molly Sims' Killer Bod 'Doesn't Come Naturally'

Molly Sims says she's definitely not one of those super slim celebrities who claim they can eat whatever they want and never work out.

The model/actress and host of Lifetime's Project Accessory opens up to Shape magazine about how hard she has to work to keep her cover-worthy physique and offers some concrete tips for getting and staying fit.

"This does not come naturally," reveals Sims, who appears on the cover of Shape's January issue looking spectacular in a skimpy bikini. "I have to work out 60 to 90 minutes at least five days a week and stick to a high-fiber, low-calorie eating plan," she adds.

Sims, 38, explains that until a couple of years ago, her fitness routine was hit or miss, and her diet philosophy was "less is more." But after constant weight fluctuations, she made a resolution that changed her life.

"I wanted to be consistent with my exercise, so I committed to working out for 30 days in a row, no matter what," she says. "For an hour every day, I did something. I was on the elliptical or the treadmill, and if someone asked me to go to a class -- whether it was spinning, boxing, yoga, you name it -- I went. By the end of the month, I felt so good, I just kept going. I didn’t want to lose my momentum."

One of her biggest pieces of advice is for people to completely commit to an exercise routine -- even if they only have 15 minutes. "Whether it's time on the treadmill or an upper-body routine, you just have to get that heart rate up and keep it there," Sims says.

For more of Molly's workout and fashion tips, pick up a copy of the January issue of Shape, on newsstands December 26.