There is no secret: to gain muscle mass the solution is to vary your workout in order to maximize long-term mass gain. Generally, it is favourable to work on strength and hypertrophy.

The 4-6 Rule: For strength, do 4-6 sets of 4-6 reps of 4-6 exercises (large exercises like Bench Press, Squat, pull-up, etc …).

The Even Rule: For hypertrophy, do 2-4 sets of 8-12 repetitions of 8-10 exercises (isolation exercises included in the program).