Is your goal to be able to pull your bow back easier and up the poundage or just to gain muscle overall? If you are just looking at being able to pull more poundage then I would concentrate on pulling and holding your bow back for longer periods of time and doing types of activities with the bowas often as possible.

If your looking to just gain overall muscle and strength, then I would say to weightlift like blookcrik suggested. Depends on what you are looking to accomplish with the muscle gain.