All mixed but handy advice.
I occasionally get it as well, and here are some good tips that I have found on the way.
Body wash with Salicylic acid (mild amount) and benzoyl peroxide (2.5%, I use 5% nightly or bi-nightly on my face to control any bad outbreaks, though any stronger or more frequent and I am asking for irritated skin).
Is pine-tar as used for skin conditions banned where you are from? It is still used here in Australia as pinetarsol (2-3% pine tar solution for skin itching from excema/chicken pox etc) however recently I found a nice artisan soap maker at a local market that makes tallow based soap with pine tar. Maybe it is just a side effect of good soap (classic tallow, no chemical additives save for the usual suspects in soap making) but I have ZERO body acne or itches after a weeks use. Plus that lovely pine tar smell

(kinda smells like a hospital to most people though, reminds me of childhood however).
Aside from this, don't scrub your back with a towel when drying off, pat dry.
Check to make sure your clothing isn't rubbing constantly on your acne prone areas of your back
Wear clean clothes ALL the time, regardless. Especially pajamas/bedclothes.
If you sleep topless, change your sheets at LEAST every 2 days, every night is better.
The last few tips are akin to facial acne, where changing your pillowcase nightly can massively decrease the level of outbreaks you get. I have never really noticed a large difference with changing pajama tops nightly, but pillow cases yes.