That is an excellent question. When I prepare for a competition myself I am always thinking in terms of trying to bring out details in my physique that nobody else has. Most guys are only concerned with the "showy" muscles like the chest, biceps, and abs. Now of course, while these muscles are the most important, there are other, less though about muscles that when fully developed, can transform a great physique into an outstanding one.
The following is a list of these muscles and some exercises that will target them:
-brachialis: hammer curls, reverse curls
-forearm extensors: reverse wrist curls, reverse curls
-serratus: pullovers, stiff arm pulldowns
-intercostals: twisting crunches, side crunches, side cable crunches
-soleus: seated calf raise
-gluteus/hamstring tie in: butt blaster machine, lower cable straight leg kickback
-rectus femoris: sissy squat, leaning back leg extension, straight leg lift
-tibialis: reverse calf raise/calf press
-erectors: hyperextension, good mornings
-upper back tie-ins: behind the neck pulldowns, seated row to chin
Add some of these exercises to your program and watch your physique take on a more complete look!