Now that you know how to target the areas you want, it's time to hit the gym. But before you head out, make the most out of your workout by avoiding these common mistakes.

"It's something that people do in just about every exercise," says personal trainer Marilynn Larkin, creator of the optimal body form program Posture-cize. "Many times people think they're working their arm muscles, but they're using momentum or hunching their shoulders over, so they're actually not working the muscles that they think they are, which can cause injury. Get someone who knows what they're doing to show you."

"A lot of times people walk into the gym and hop on the first piece of equipment," notes Alyssa Shaffer, author of "The A-List Workout" (McGraw-Hill). "Have an idea of what you're going to do before you even walk in the door. That way you can give yourself time to finish what you have to do."

Lifting weights that are too heavy can damage your joints. "Guys in particular tend to work too heavy," says Larkin. "They think, 'I can do 10 pounds, I'll do 20 pounds on my next set of repetitions.' They want to work the muscle, but there are joints there, and the joints can't

support that amount of work."

"Some women work too lightly, not wanting to progress because they're afraid they're going to get big, which does not happen because that depends on hormones and genes," says Larkin.

"It's really important to wear the right shoes for the right activity," says Shaffer. "If you want to run or walk on the treadmill, invest in a pair of running or walking shoes. If you're going to be taking a lot of kickboxing classes or classes that involve a lot of moving from side to side, go for a pair of cross-trainers. An old cotton T-shirt isn't always the greatest choice because it absorbs moisture."

"If you're paying a personal trainer, make sure that you're working out during that entire time," Larkin says. "There are ways to work out where you're alternating an upper-body and a lower-body part so that your body rests in between but you're still in motion."

"It's good to have a familiarity with things, so ask someone on the floor if you don't know how to use something," Shaffer says.

"That really sets you up for injury," Larkin explains. "You're overworking the same muscle groups and not strengthening the other muscles. People want to work the biceps, cleavage, what they can see in the mirror. For good posture, you need to work the back muscles. Change your workout every four to six weeks."

"People tend to feel silly [seeking help]," says Shaffer. "They think, 'I can't afford a trainer so I can't possibly ask anybody for anything.' Most gyms keep a steady crew of people on the floor to help."

"Whether it's a dance class or a yoga class, the idea is to get over yourself and give yourself an opportunity to try new things," Shaffer says.