I don't think there's a whole lot you can do for your back without proper equipment. Your abdominal muscles are important in torso/back/spinal stability (in conjunction with the muscles of your back, of course), so doing ab exercises will help a bit. Bent-over rows can be performed with just dumbells and a sort of bench (if you don't know what this exercise is, just go on YouTube or Google and you'll see what I mean by bench), and they work most of your back muscles (latissimus dorsi, lower and upper trapezius) as well as shoulders and arms. I can't really think of anything else that targets your back that can be done with minimal or no equipment. But you could, as I said with abdominal exercises, work to strengthen your core, which might help some.