The best manner in which to gain muscle size is to use a cycle. A training cycle is a weightlifting program designed for a specific reason for a specific period of time. It is not wise to use the same program all of the time because the body can adapt to it and then cease to grow. When this happens, boredom sets in. Injuries are also a possibility when training is monotonous and nonspecified.
Cycling provides the mind with a target to aim at, along with plans on how to get there. Cycles provide purposes for the training approach selected. Since there are different types of training for building muscle size, a cycle allows you to try one approach, check it out and see if it works for you. Some of the various training cycles will be provided and the concepts behind each will be looked at. The best approach to a training cycle is:
Plan - Plan out your specific training cycle.
Perform - Perform the specific cycle in the gym.
Check - Check your progress or lack of progress.
Adjust - Adjust your cycle to your unique physique.
Reperform - Perform the cycle with the necessary adjustments.
Critique - Critique the cycle to see if it gave you what you wanted. If so, use it for a longer period of time, then put it away for awhile, and use it again in the future. If not, get rid of it.
If you find that a certain training cycle is especially effective in packing on the muscle mass, put it down on paper and don't lose it. Use it again after taking some time off. The body adapts to challenges so you don't want to continually stay at the same place. The best way to force positive change is to use variety in your training. If you can put together a string of effective cycles you can make your body do what you want it to. A good cycle is an important discovery for a person who wants to gain muscle mass.
You can use the same approach with your diet. You can cycle a certain diet for a period of time and find out if it is effective or not. Use the cycle approach listed earlier (plan, check, etc.) with your diet to make it conform to what you want.
Calibrating and fine tuning is part of the art and science of gaining muscle mass. Make good use of notes, keeping several notebooks where you can use them on a consistent basis.