i've started using my alep soap on my body too, and it hepls my acne on the back. i suggest you just take care of your back as you do with your face, wash and scrub, then apply a specific acne cream.
i find this head&shoulders tip quite interesting as i read once zinc was effective for acne. i'm not sure i'd buy the shampoo (that i don't like much btw and as i don't have dandruffs i wouldn't have any use for it) but try to find products containing zinc.
and if pimples appear on the areas where are your bras straps, i suggest you change more regularly of bra and wash them, they may carry some bacteria and sort of help redistribute it.
the neutrogena products are i think the best you can find at the supermarket, you can also try them.