Having just celebrated her 25th birthday, Rihanna has a lot to celebrate. Aside from a chart-topping career and a new fashion line, the singer can toast to being in the best shape of her life -- and it certainly shows when she's wearing those teeny-weeny bikinis. "If you want to work the butt, you've got to work the hamstrings," says Rihanna's trainer, Ary Nunez. Even when the two aren't working out together, Rihanna will do sets of high knee standing crunches to tone her abs in the hotel room. The trick is to space out your workouts throughout the day and the week so that you don't get bored.