Train Your Core
Your "core" muscles include some muscles in the abdomen, back, pelvic floor and hips. Their jobs are to support your spine, maintain your balance and move your legs powerfully -- factors that are all crucial in such sports as running, tennis, basketball, skiing, and more.
When we have weak core muscles, it's much harder to maintain good posture; if we get tired, we start leaning forward as we move -- poor form that can lead to muscle strain and decreased lung capacity.
Pilates classes are one of the most favored methods of strengthening the core muscles; some kinds of yoga can also help. If you prefer to strength-train with weights, ask a personal trainer which exercises can work your core best.