When it comes to whipping your body into shape, we can all take a lesson from Janet Jackson. In fact, the 49-year-old singer, who is currently on her Unbreakable world tour, is proof that a flat, toned stomach is not just a trait of 20-somethings. Her secret: the Cross-Legged Reverse Crunch. "This is one of Janet's favorite lower ab exercises," says her longtime trainer, Tony Martinez. It works the hips and core and gets her the definition in those sexy abs she's known for." Do it 2 to 3 times per week to see results in as little as 2 weeks.