Shooting your bow is best, but if you're like me and can't shoot often, you may want to work out a bit.
I do the following exercises, once or twice a week, with at least a day's rest before working out again or shooting:
-Bent-over rows (for upper back muscles)
-Upright rows or shrugs (for trapezus)
-Arnold Press (for deltoids)
Start out with light weights, build endurance, and then move up to medium weights. Don't go ultra heavy so as not to injure yourself or get bulky muscles with less range of motion.
I just use two 25 lb dumbells. Do some situps and pushups too to exercise your core muscles - you use those for pretty well everything anyway.
There's also an Archery Fit DVD on 3 rivers archery site - that might be a good purchase.