South,
S.C. gave a pretty good answer. Most folks don't really need to increase their back strength, they just need to learn to use it correctly. Many tend to be unable to allow the string arm to be as relaxed a possible and to move their string shoulder to the back. Most want to "strong arm" draw rather than use leverage. This is an area where the "form master" can help you learn the feel.
It is also quite difficult to LEARN to do this with a hunting weight bow. As S.C. mentioned, Ranger B (Jimmy Blackmon) has a good video out called " finding your back" (I think it's called). I also have done a couple videos that show the shoulder movement, one, the rubber band video shows at length the shoulder movement you are looking for.
Using that shoulder movement to "lever" your upper arm bone into the bow, once learned, really helps when you want to shoot a heavier bow. The movement is just very hard to LEARN on a heavier bow. Even with just the rubber band, you WILL feel it in the morning but it doesn't take very long before shooting 50 or more pounds will be pretty easy. Again, not really a matter of strength but of leverage and learning the shoulder movement required.
Arne