I try to do three back excercises on back day, pulling from an upper, mid, and lower direction using varying grips (side/flat/underhand as well as close or wide). Always spend a good 5-10 minutes stretching out first.
Typical back day (3 or 4 sets each depending on whether I'm bulking or cutting):
Bent Rows (bar, wide grip):
1 warmup set 10 reps/light, followed by three pyramid sets.
Seated rows (close grip, hands facing each other): 3-4 pyramid sets.
Cable pulldown (shoulder width grip, underhand): 3-4 pyramid sets.
Other Back Stuff:
DB rows on flat bench
1-arm seated rows
Straight-arm pulldown
Pullups