1. Set realistic exercise goals. Set goals that you not only know you can achieve and are specific, not vague ("I'll cycle twenty miles this week," not "I really should get more exercise this week.").

2. Whatever activity you pursue, don't overdo it. The most

common cause of injury is exercising too aggressively. The "too much, too soon" syndrome. Start any new exercise at a relatively low intensity and gradually increase your level of exertion over a number of weeks. Use the "10%" rule: In general, don't increase your training load the length or frequency of workouts, the intensity, or the distance by more than 10% a week.

3. "No pain, no gain" is a myth. Exercise should require some effort, but pain is a warning sign you are foolish to ignore. If you have continuing pain during an exercise, stop and don't continue unless you can do so painlessly. (If the pain occurs in the chest or neck area, you should contact your physician immediately.) General muscle soreness that comes after exercise is another matter: It usually indicates that you are not warming up sufficiently or that you are exercising too long or strenuously.

4. Control your movements—if you can't, slow down. Rapid, jerky movement can set the stage for injury. As you move your limbs, keep the muscles contracted and move them as if you are pushing against some resistance.

5. Watch your form and posture. In most activities, stress can result from poor form. Keep your back aligned (abdominal muscles contracted, buttocks tucked in, knees aligned over feet). This is particularly important when jumping or reaching overhead.

6. Don't bounce while stretching. This "ballistic" stretching can increase the chance of muscle tears and soreness. Instead, perform "static" stretches, which call for gradually stretching through a muscle's full range of movement until you feel resistance. This gradually loosens muscles without straining them.

7. Use good footwear. Wearing improper or worn-out shoes places added stress on your hips, knees, ankles, and feet—the sites of up to 90% of all sports injuries. Choose shoes suited to your activity and replace them before they wear out.

8. Avoid high-impact aerobics. Most aerobics instructors and many students suffer injuries to their shins, calves, lower back, ankles, and knees because of the repetitive, jarring movements of some aerobics routines. Substitute the marching or gliding movements of low-impact aerobics for the jolting, up-and-down motion of typical aerobics.

9. Warm up and cool down. Slowly jog for five minutes before your workout to gradually increase your heart rate and core temperature. Cool down after exercising with five minutes of slower-paced movement. This prevents an abrupt drop in blood pressure and helps alleviate potential muscle stiffness.

10. Replace fluids lost through sweating. This is particularly important in hot weather, when you can easily lose more than a quart of water in an hour. Neglecting to compensate for fluid loss can cause lethargy and nausea, interfering with your performance. Even if you don't feel thirsty, it's important to drink at regular intervals when exercising. (Thirst is satisfied long before you have replenished lost fluids.)