The old saying of "use it or lose it" is quite applicable here. Resistance training is the answer when it comes to maintaining (if not gaining) muscle mass and strength. A recent study published in Journal of Strength and Conditioning Research (2011, volume 25, 1-9) has indicated that age-related sarcopenia (muscle loss) is not inevitable as long as resistance training occurs. The resistance training promotes the release of insulin-like growth factor 1 (IGF-1), a hormone associated with muscle growth. If you have been sedentary for a while, the recommendation is to begin a gradual training program of weight lifting. You will discover dramatic improvements in muscle mass and strength gain. In addition, you'll feel more energized, be able to sleep much better at night, and have a more upbeat attitude!
Consider increasing your protein intake (e.g., whey and casein protein post-exercise) to accommodate the increased muscle mass. But don't go overboard on increasing your protein intake as too much may affect kidney function and cause kidney stone formation. Good sources of protein include chicken, fish, milk, eggs, yogurt, beans, and peanuts.
The Institute of Medicine (IOM) has recently recommended that adults over age 50 should get 1200 mg and 800 IUs of calcium and vitamin D, respectively. These nutrients help maintain bone density and muscle health. Vitamin D has been found to have antioxidant properties and may reduce the risk of cancer, heart disease, autoimmune diseases, and diabetes.