I agree, more hens would help. However, this problem may never go away completely. Right now I have three roosters and 77 hens, and each rooster still has his favorite girls that get a bit more wear and tear than the others.
There is some thought that the wear and tear is genetic--that hens that get really bald don't have feathers that are as strong, or some similar reason. Personally, I do not know if that's true or not. What I do know is that it's going to be a byproduct of having a rooster in with your hens, and the best thing to do is relax about it. People seem to care about bald chicken backs MUCH more than the chickens themselves do. I had a chicken go all last winter with a completely bald back and it didn't seem to faze her one bit.
If it really bothers you, you can put chicken saddles on them. As I said, it doesn't seem to stress the hens so I don't worry about it.