Proactive seriously works. I've used it!
Anything that can be used for acne on your face can be used on your back. The good thing about treating your back is not being self conscious about flaking skin as you would be if it was on your face. Make sure you can clean all of your back with ease. The sponges on the brush are nice. There are acne body washes. There are also lotions and ointments for the back as well. If it was me, I would use a cleaner with sialic acid. Then I would use an astringent once I got out of the shower and dried off. Then I would use a spot treatment that is 10% micro benzoyal peroxide just on the areas that are broken out twice a day.
If your back gets tight or itchy from dry skin use a lotion.