Unless you were born with one, no amount of exercise will give you a rear view like Jennifer Lopez's. But training your butt muscles can make them leaner, tighter and firmer, says John Platero, founder of Los Angeles-based Future Fit, a company that trains and certifies personal trainers. Besides looking great in a pair of 501s, strong glutes are a big plus in sports involving running and jumping, such as basketball, volleyball, soccer and kickboxing.

Despite conventional wisdom, improving your rear view involves strengthening not one but three important muscle groups. In addition to your gluteus maximus (the largest and outermost muscle), you need to work your hamstrings (the backs of your thighs) and your erector spinae (which support the lower back), too. "The muscles of the posterior chain work together as a team," says Platero. "Your glutes will look and perform better if you have powerful hamstrings and a strong lower back -- and a weakness in the chain could cause an injury."

Platero has picked his top moves for nailing three associated muscle groups. For these, Platero likes the lean look created by doing more reps with lighter weights. The glutes are strong, he says, so they can handle extra reps. And instead of getting big, they'll get tighter and take up less space.