http://www.acne-web.org/scrub-acne/

What’s your preffered back/chest acne scrub?
I have alot of acne on my chest and a little on my back. Can you please tell me some good scrubs, rinses, washes, etc. To try for it?
When speaking about back acne in particular, there is a particular form of acne called “acne mechanica”—a fancy name for acne which is caused by irritation of the skin. Anything that repeatedly rubs against the skin, such as backpacks, rough massages, tight fitting clothing, weight lifting machines which press on the shoulder area, or anything else that rubs the back area and irritates the skin can aggravate acne in that area. Acne mechanica tends to be aggravated by moisture, so if you’re sweaty and combine that with the repeated rubbing that comes with your backpack, clothing, etc., that could make the problem worse. By simply existing as a human being, your back is going to be in contact with chairs, clothing, beds, and other people’s touch so no one will be able to remove all irritation of the skin of the back. Still, there are some simple ways you can help the situation.
Some things you might want to consider”
Avoiding irritation Try wearing breathable cotton clothing more often.
Try carrying your belongings in something other than your backpack while treating your back acne. Once you’re clear, you can try using your backpack again. If you stay clear, great. If not, perhaps a backpack isn’t the best option for you.
Generally try to be aware of anything that irritates the skin of your back and make accomodations if you can.
Try to time any daily treatment of your acne so that you can shower and treat your skin after a sweaty workout.
And check here for information about back acne.
http://beauty.about.com/od/bodybeautiful/f/backacne.htm
DIY Face Scrub (Fade Acne Scars & Age Spots)