How do I gain muscle mass?

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.

Ingest lots of protein-rich foods and exercise with heavy weights for more than 30 min daily.

Human Growth Hormone. HGH worked for everyone in the Mitchell Report, it can work for you.

I’ll echo doscrash. It’s the rebuilding that occurs in between workouts that builds muscle mass. Protein, which is broken down into amino acids, is what goes into muscle construction. Foods with the highest concentration of bioavailable (i.e. absorbable) protein (in descending order) include: whey (the liquid at the top of your yogurt), whole egg, milk, egg whites, whey protein powder, very lean and lean meats (shrimp, turkey, salmon, chicken, scallops, lean beef, tuna, buffalo), soy crumbles, and kidney beans.

On the workout side, you’ll want to approach muscle failure every time you lift, meaning you want to add weight (increase effort) until you reach a peak and then keep lifting by subtracting 5 or 10 pounds on subsequent sets until your arms, for example, are jelly and you can barely curl three pounds. I’m exaggerating a little, but the idea is what you should shoot for within your own routine of sets and reps. Body for Life describes this method (Set 1 @ 20% effort, increasing to Set 5 at 100% and then doing an immediate sixth set with a slightly different exercise using the same muscle group).

Lift weights on regular basis, high protein diet, enough rest and sleep.