How to gain muscle mass!

1) Use heavier weight than the last workout for the same exercise for the same amount of repetitions and same rest periods;

2) Use the same weight as your last workout for the same exercise but for more repetitions using the same rest periods;

3) Use the same amount of weight and repetitions as your last workout but do the exercise is less time;

Taken from : http://www.building-muscle101.com/how-to-gain-muscle-mass.html