When you understand the "science" behind how and why the body gets stronger and bigger, you can apply it when you choose to do so.
The body is a mechanical and organic energy that "grows" and becomes stronger when it is under "PROPER" stress and work.
The science is simple. But do not make the common mistake to underestimate the science just because it is simple. Simple things are usually the most powerful things.
To grow, you must take in more calories, lift heavier, and sleep as much as possible. The body needs more nutrients as you break it down with hard training. The body needs heavier loads in order for it to be forced into growth and strength. The body needs enough rest in order to restore and build.
Take in at-least 3,000 clean calories per day, if you can not do that everyday, do it at-least on your workout days. Clean calories are calories NOT from fast food or junk foods. Vegetables, fruits, and lean lean meats. If your calories are NOT clean, be ready to add a lot of fat to your body.
Lift heavy, work 100% effort every time you workout. Keep reps within the 1-7 rep range, warm-up with a light weight for about 30 reps at 2 to three sets. The warm up is important so that you will NOT tear a muscle or inflict some other kind of injury. ALWAYS use proper form, some exercises allow a little room for cheating, but normally use VERY STRICT form.
DO NOT work out more than 45 mins to 1 hour if your goal is for size.
Long workouts mean you need more energy, the energy your body requires will cause your body to seek that energy by burning possible muscle tissue, long workouts also lower testosterone levels.
Stretch after each workout, stretch until you feel very loose and relaxed all over the body and through it. A stretched and flexible muscle grows much faster than tight stiff muscles.
Drink PLENTY of WATER, almost a gallon a day if possible. The body grows faster and suffers less energy loss when it is hydrated well.
Breathe deeply. Simple: Breath is "life". Breathe OUT when exerting force.
Eat more green vegetables, lean protein, and lower carbs. make sure to eat plenty of GREEN VEGGIES. Green veggies clean the body and make it stronger supplying vital nutrients. The CLEANER the body, the faster it RECOVERS and builds.