You want your bow arm shoulder to stay in the same position it is in when you're just standing in a relaxed position. I keep it in that position by pushing toward the target. The result of the pushing is that my shoulders rotate in toward the bow, my arm extends, and the bow and arrow move in towards my body. Don't overdo the pushing; you just want to push enough to keep your bow arm shoulder down. For me, this is pretty easy to feel, unlike my back muscles and moving my head, which don't seem to be connected to my brain as much as I would like.