Gain muscle mass by applying stimulus
This is simple, you need to workout. You don't build muscle in the gym, you destroy it. There are some late night commercials that want you to believe that it only takes 5 minutes a day a few times a week, for most people this simply isn't the case. To gain muscle mass you need to hit your target muscle group hard. That old saying 'no pain - no gain' applies here. Workout the target muscles until it hurts, then move on. This is an important step if you want to get huge.
Gain muscle mass by increasing your protein intake
It is generally accepted that you need to take in 1.5 grams of protein per pound of weight to properly gain muscle mass. If your body doesn't have the building blocks it needs to gain muscle mass it won't, no matter how much you workout.
Gain muscle mass by producing testosterone
Let's face it, as we get older we produce less testosterone, it's just the way it is. If your a bit older then take bodybuilding supplements to increase your testosterone levels. No matter what your age make sure you take a good quality multi vitamin designed for gaining muscle mass. The Animal Pak is a great one. Keep in mind that a standard daily multi vitamin is formulated for 'normal' people, you aren't normal, you workout hard so you need more than standard, a good vitamin pack designed for people that workout is key here.
Gain muscle mass by gaining weight
I know you can't believe this, but here's some numbers for you. If you want to gain an inch of muscle mass that's about 20 pounds of lean muscle. So, you need to gain 20 pounds. Make sure you take in enough calories to gain about a pound of weight every week. So that means if you gain a pound a week it'll still take 5 months to build an inch of muscle all around. I really wouldn't push the weight gain much over 1.5 pounds a week, any more than that and you will start to add fat as well.
Gain muscle mass while you sleep
As I said earlier, you destroy muscle in the gym and you build it while you sleep. If you've taken care of all the earlier requirments to gain muscle then this is the last thing you need to do, get plenty of sleep. If you don't sleep enough you'll never gain muscle mass at your fullest potential.
These are the most basic things you can do to get huge, if you want to kick it up another notch I suggest learning about the top rated muscle building supplements as well.
Gain muscle mass by following the above steps
And finally, make sure you consult with a physician before you start any diet or exercise program. Just follow the above rules and I guarantee you will quickly gain muscle. You will also find many useful links right here on this page to help you gain muscle quickly.