A gym has a finite number of machines and even with free weights there are only so many movements, so the best way to shock a muscle and mix up your routine is by changing the number of sets and repetitions. To develop any muscle you have to work it from 3 angles or sides, but instead of conducting the cliché 4 sets of 4-8 reps per exercise, try mixing in the following: 10 sets of 10 reps, 10 sets of only 1 rep (using maximum weight), going up and down the rack or stack (starting with the lightest weight and going to your max and then back down), simply going down the rack, or sets of 100 reps.
Here’s a sample shoulder workout: Seated overhead dumbbell press – 100 reps (this will warm you up and burn you out:) Side lateral raises 10 sets of 10 reps. Bent over side lateral raises 10 sets of 1 rep using the heaviest weight you can manage. Then finish with pull to chins starting with the lightest bar on the rack and working your way up to the heaviest you can manage for one rep and then back down the rack. In summary, be creative in your routine, change the exercises with each workout, always work a muscle from 3 angles and remember – get crazy with repetitions and sets.