Well, I also have back/chest/shoulder acne and I really hate it. I have been using tee tree oil for about a week and I have seen improvement. I've also been using a Cedar Oil facial peel product on my back and chest and it clears all the dead cells away very nicely. My skin feels much smoother after I use it.
Last summer I was in a normal beater shirt and my girlfriend's friend said something like "Wow John, you have a lot of mosquito bites on your back." Of course it was embarrassing but Im guessing she never knew you could get acne on your back. Im hoping this summer it'll be at a level where im comfortable going to the beack without my shirt on.
Best of luck... you are certainly not alone!
Im also using a tee Tree Oil Face wash now by Thursday plantation.
Also, people have said using head and shoulders shampoo as helped them clear their body acne.