One thing I do when the season rolls around (I have NO formal bow training......nor do I know if this is a great thing.....it just works, for me)....is use hand weights on the treadmill. One of the treadmill programs I utilize starts out with a warm-up session. During this warm-up.....I put 4lb (you could use lighter or heavier...depending on your strength) hand weights to go through a pretty vigorous warm up (walking on 4 to 4.5). I'm a former college baseball catcher that's had some shoulder issues.....and I can tell you that the muscles you're trying to strengthen (I'm talking ONLY about the muscles needed to stabilize the BOW hand)....aren't very large muscles....and they can't handle a lot of heavy weight lifting (w/out damage). Small weights and lots of reps (arms pumping....in my example) seem to work well. Having never shot a bow before last year......but having always been an athlete.....I thought the draw weight of the bow might beTHE big challenge. I gained draw weight very quickly.....but the weight of the bow.....for long practices.....was a challenge. Lifting light weights (as described) has helped a lot.

Good luck.

Jeff