With your doctor's OK, get 20 to 30 minutes of aerobic exercise -- the kind that gets your heart pumping -- on as many days of the week as you can. Avoid anything that puts a lot of stress on your joints, such as running. Some good activities to try include walking, water aerobics, swimming, and bicycling.
A couple of days a week, do some strength training to build up the muscles that support your joints. Make sure you also stretch to make your body more flexible -- but be careful not to overdo it.