Just keep it in the position it normally is in when you aren't pulling a bow. Raise your arm up without a bow in in and see where your shoulder is; that's where it should be when you draw the bow. Then scrunch it up toward your neck; that's where it shouldn't be. It's not that difficult of a thing to get right if you're aware of it. It's just that when we're not aware of things, they get out of whack. You have to exert some muscular force to keep your shoulder in a natural position when you're pulling the bow, otherwise it scrunches up toward your neck. For me, that feels like I'm pushing with my bow arm. You just don't want to push so much that you're hyperextending your shoulder unnaturally in the other direction, either.