Don't slouch. Perfect posture is crucial to injury-free and effective strength training. "No matter what exercise you are doing, you want to tuck your hips slightly forward, engage your lower abs, and keep your ribs lifted," says Diane Isaacs, a Los Angeles-based personal trainer and Ironman triathlete. "Not only will this let you work more muscles than the ones you are targeting but it will also give you more stability. When your body is stable, you will be better able to lift the weights with a controlled, fluid motion, rather than a jerky one, which can cause injury."