2. Forgetting Your Core

The "core" refers to all the muscles of your torso and pelvis — the area of the body that supports the spine. Like the trunk of a tree supporting the branches, these muscles help stabilize your body as it moves, including during exercise.

Solution: Tighten your abdominal muscles as you exercise; in other words, "engage" your core. A strong core lays the foundation for muscle-building, balance and coordination, while helping prevent injuries such as back problems.

3. Overdoing It

As an athlete, I always tried to push the envelope when it came to increasing my performance. But like every other hard-training athlete, I overdid things from time to time and so succumbed to a physical condition known as "overtraining."

When you exercise, you tear your body down to some extent, and it needs sufficient time, rest and nutrition to recover. Overtraining happens when you push your body past the point of a full recovery before your next workout. Signs of overtraining include excessive fatigue, insomnia, decreased performance, nagging muscle or joint pain, and more frequent illnesses and upper-respiratory infections. If I have no energy the day after a training session, I know I've completely shot my body.

Solution: Make it a goal to work out for roughly 45 minutes, 3 to 5 times a week, and be sure to let your body recuperate between workouts. Doing more and more — without building rest and recovery into your routine — does you less and less good. Instead, gradually increase the amount of weight you use, and the time you spend doing cardio.

Finally, and most important, listen to your body. It will tell you if you've overdone it.

4. Using Poor Form

I see this workout mistake all the time. Many people who lift weights, for instance, rely on momentum rather than the power of their own muscles and so jerk the weights up or down. This is hard on your joints and raises the risk of serious injury.

Solution: Select a weight that lets you do 10 repetitions slowly. Raise and lower the weights slowly, concentrating on the power of your muscles to do the work. Your 10th rep should be difficult enough that good form becomes hard to maintain.

As for cardio machines, here's another of my pet peeves: resting on the rails. This negates a lot of the aerobic benefit of the machine, and could hurt your wrists and back. So don't lean too heavily on the stair-climber or treadmill rails. Stand straight with good posture and lightly rest your hands on the rails for balance. If you find yourself hanging on to the rails, you may need to lower the speed, the incline or both in order to regain the proper posture.

Try correcting these mistakes, and you'll be surprised at the progress you make!

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Go to the AARP home page for tips on keeping healthy and sharp, and great deals.