Hi! I had back acne for about two years. I think it started because I used to go workout and not shower right after. At first, the acne was only on my upper back but then it started to spread to my lower back as well. I would label the acne as moderate; at times there were only a few active pimples, but other times I would have enough to make laying on my back uncomfortable. I also had an area where there was just a lump of "dormant" acne (or at least that's what I think it was?). I tried a few drugstore products like the neutrogena body wash, charcoal soaps, natural soaps, scrubs toners, but they did not work. However, I stumbled upon the Mario Badescu Aha Botanical Soap about 5 months back and so much has changed. I am so glad I found this product because It has made my back almost entirely clear. I used the soap with an exfoliating washcloth and it really helped with resurfacing skin and drying up acne spots. It is also pretty affordable which I am very happy about. You can purchase a bottle at Ulta for $8 for 8 oz. which is great if you want to try it out. I started noticing results within two weeks of using it. Now, I am just left with acne scars, but I am no longer as self conscious about my back. 
I hope my story helped and I wish you luck!
This what helped me. Please don't use home remedies because it can be more harm than good!
My routine helped me with chest and back acne. http://www.acne.org/messageboard/topic/363156-how-i-got-rid-of-bacne/