Packing on size is easy, as long as proper techniques are applied. Keep in mind that everyone has a different body type.
Here are some tips to help you gain muscle mass.
#1 Weight Train
Train with heavy weights using less reps. Using appropriate weight, you should only be able to do 4-8 reps. Using heavy weights and low reps puts your muscles and nervous system under much more stress than using lighter weights for many reps. The rest time in between sets should be approximately 2 to 3 minutes.
#2 Eat more Calories
To gain mass, you should strive to eat around 18-20 times you body weight in calories. The extra calories are needed to build new muscles and to repair muscle tissue that is damaged during the heavy workouts. However, do not consume empty calories; go for foods which provide you with correct nutrients essential for gaining muscle.
#3 Eat more Protein
Only diets high in protein help build muscle when incorporated with intense physical training. Without protein, your body will not be able to build new muscle. Carbs serve mainly as energy for the body where as protein provides the necessary amino acids to build and repair muscle. To build muscles get at least 1 gram of protein per pound of body weight.
#4 Eat Fat
Dietary fat has a direct relationship with testosterone production. An increase in dietary fat intake seems to bring on an increase in testosterone levels, which is a must when you are trying to gain mass. Your main focus should be to increase your intake of essential fatty acids and not saturated fats.
#5 Drink more Often
To make sure that your muscles stay hydrated you must drink plenty of water. Dehydration can happen easily if you train hard. A dehydrated muscle takes longer to repair itself than a hydrated one.
#6 Take a Multi Vitamin
If you want to gain muscle, you must make sure that you are not deficient in any vitamin or mineral that your body needs. So it is a good idea to take a good multi vitamin tablet.
#7 Stop all Aerobics
In order to put on serous mass you need to stop all of your aerobics. If you do need to carry with it remember to limit it to once every week or once every two weeks.
#8 Take ample Rest
Resting the body is just as important as dieting and training. Try to get about six or eight hours of sleep a day. Resting is what actually causes the muscles to grow. Taking days, even weeks off from training will only benefit you.
Be patient and consistent. You may have the best diet, the best training schedule, join the best gym but without consistency it's all worthless.