You have to lift weights on a regular basis, enough so that you consistently tear down your muscles and rebuild them. The only way you rebuild them, however, is if you have proper rest for 24 hours after your workout. If you want to build muscle mass quickly, lift weights and cut down on your fat, but increase your protein intake. The rule of thumb for building muscle mass is to eat your weight in grams of protein per day. For example, if you weigh 150 pounds, each 150 grams of protein.