Kristin Cavallari has always had an amazing body. She celebrated her birthday at The Bank Nightclub on Saturday (15th Jan). I’m such a fan of the dress!

Kristin Cavallari’s workout mentality? No pain, no gain.

The Hills star, 23, takes a circuit training class at her gym that “kicks your ass,” she tells UsMagazine.com.

“I never want to go and I walk out dripping in sweat and it kills you, but it works,” she admits of the class, called Burn 60. “And I feel good… This is the first time in my life where I’ve really been working out consistently.”

“Once you see results, you want to maintain them and you want to keep it going and that makes you want to go to the gym,” she tells Us.

As for her diet, Cavallari’s secret to staying slim and trim is all about control — and a reward!

“Control for six days of the week,” she says, “and then letting yourself eat whatever you want on that one day!”