I agree with Melinda, doing other activities will really help you to muscle your back and to have a better position when you ride.
Remember to always keep our shoulders rearwards and to look far ahead!
The best way is doing some different activities from riding to develop your back's muscles. As swimming, riding bikes, or even yoga ! If your back is muscled, it's going to be easier to stay straight on your horse. You also have to be careful about where you look, it'll help you to stay straight :)