Rather than being a weekend warrior, work to maintain a moderate level of activity throughout the week. Whatever sport you are playing, learn to use proper form and wear safety gear if appropriate. Remember that you may not be as agile you once were. Learn to accept your body's limits. This may require that you modify some activities.
When exercising, increase your level of activity gradually. Make sure that you are getting a good balance of exercise, including cardiovascular, strength and flexibility. When doing squats, do not bend your knees below 90 degrees. When stretching, keep your feet flat on the ground as much as possible to avoid twisting your knees.
When you are jumping, whether while paying sort or during exercise, be sure to land with your knees bent. Always warm up before exercising or playing sport. This should include some form of dynamic stretching. Cool down afterwards. Be sure to wear properly fitting shoes when engaged in physical activity. This will improve shock absorption and stability.
Try to use the softest exercise surface that you can. Avoid running on hard surfaces such as asphalt or concrete. Always try to run on flat surfaces. Running uphill may place overdue stress on the Achilles tendon and the leg itself.