I used to have moderate to severe back acne until I started using the Glytone Back Spray, which I found on amazon.com. It is 2% salicylic acid, and I spray it on my back after getting out of the shower. My back is so smooth and clear now. The spray bottle is designed so that you can spray the bottle yourself (when you turn it upside down, it is still able to spray).
I tried all kinds of body washes that contained salicylic acid; they would work for a while, but then my acne would re-appear.
Now, I just use a benzoyl peroxide wash (10% is okay for my back, not too strong) in the shower for my back and cleanser for the rest of my body.
Hope this helps!