The most important principles you can follow to pack on muscle mass are muscular tension, metabolic stress and muscular damage.
# Power Principle 1: Muscular tension
Muscular tension is not about using the most weights in the gym but squeezing the muscles you are working on.
An example: When doing a flat bench press, focus on your chest while you try and squeeze the bar inward on the way down and on the way up. This will activate more muscle fibers in your chest as compared to just blindly lifting a ton of weigh up and down.
# Power Principle 2: Metabolic stress
Exercise scientists have known for years that muscle growth occurs due to metabolic stress which is associated with the burning feeling when you train a muscle group.
An example: After doing your multi-joint exercises such as squats, rows and presses finish off your workouts with pump sets on cables or machines. Go for the burn! Using single joint moves will subject underworked muscles to stress which will improve muscular symmetry. Muscles need to be worked across a wide range of angles and planes.
# Power Principle 3: Muscular damage
The simplest way to induce muscle growth is to do something that is unfamiliar, focus on the eccentric (lowering part of an exercise) or by stretching a muscle while it is being activated.
An example: Try and push one more rep, add a set or add weight to the bar.