Start heavy and progressively decrease resistance, to shock the muscle.

FIT TIP: Breaking plateaus....
Every other week, pyramid your weight down every set, starting with your heaviest weight on your first set (after a sufficient warm-up) and progressively decreasing your weight and increasing your reps through your last set. In addition, perform a session of drop sets once a week for a month do an exercise to failure, decrease (“drop”) the weight, then keep going until you fail with that weight; continue in this fashion until you’ve finished all sets.