I would also say that shooting a lot is the best way to build the shooting muscles up. I'm 23 and if I haven't shot in awhile even I start to get a little shaky after about a dozen shots. Besides shooting you should try working out a little bit. You don't even need weights or a gym, I would start just by doing some push ups (some with your hands in tight to your body, others with your hands spread out wider...works different muscles) and some pull ups (really concentrate on using your muscles in your back and not just your arms). In short order that 40 lbs will start to feel like a breeze to pull back.