To warm up before my chest, shoulders, and triceps routine, I take extra time on my shoulder girdle. One exercise that I do is using very light weight dumbbells, I do a push-pull, mimicking drawing my bow. I do 4 sets of 10 reps for each arm, alternating pushing and pulling. I also do chest flyes with light weight dumbbells. Basically, like trying to hug a tree, elbows bent, and a nice controlled stretch before "hugging". Slow, steady, and controlled are vital to keeping your shoulders strong.