I apologize for the length, but I hope it helps.
One thing I always told myself was if I ever came up with a routine that works, I would post it on this website. Acne.org has been my go to for over 6 years, but unfortunately I've never had any luck after reading countless threads. I'm currently 31 (white, male, 170lbs, 5'11) and I've had back acne since I was 26 (no facial acne). The acne was all over the place and some was cystic in the lower back. Consistently over the last 4 months, I've been down to 1 to 2 manageable spots from what used to be anywhere from 20 to 30 at a given time. Oddly, I had acne when I was young 13-17, but it was definitely the hormonal, you'll grow out of it kind. I'll explain my theory to what may have caused my adult back acne in a bit...
Prior to getting back acne I lived with my shirt off during the summer and had no problem walking around the house. Over the first year or so of getting back acne I started becoming embarrassed/insecure. I wouldn't take my shirt off in front of my wife and during the summer and I would wear a cutoff rash guard (water shirt). If you know what I'm talking about, it blows and I feel your pain, but there is hope.
If you can name an over the counter product on this website outside of Dan's formula (which I'm sure is great), I've more than likely tried it. I never went to a dermatologist. I'm cheap and have a different point of view on spending that kind of money on something you have to stick with long-term. I'm sure opinions will differ and that's perfectly fine.
The list of items I've tried:
Juicing, Apple Cider Vinegar, Alpha Hydroxy Lotions, Lemon Juice, name brand body washes, baby powder, calamine lotion, alcohol, hydrogen peroxide, pantothenic acid (B5), multivitamins, zinc, proactiv, fish oil, different laundry detergents, tanning, loose clothing, 2 showers a day, and on and on. (ridiculous and frustrating being 31 and married.)
From the above list, I would allow for 4 to 6 weeks to let my body go through the purging phase, then recovery. Some of the stuff actually made a difference like lemon juice and calamine, but temporary. My goal has always been to use something that's cheap and requires a non time consuming routine. It just never seemed logical to have to spend a crazy amount of time or money doing these things when we shouldn't have to do anything at all and be perfectly fine, which I've tried.
Here's what's worked for me and I sincerely hope it does for any back acne sufferers reading this.
Slow the picking down. This was honestly the hardest habit to break. I approached it kind of like someone would quit smoking, which ironically I quit 5 years ago (hmm). I went from picking 2 per day the first week, to one the second week, every other day, to none. Commit to this by either mentally beating it in your head, writing it down or whatever it takes, but slow it down immediately. If you do pick, only do it right before taking a shower to wash away the bacteria. If you have one pop, soak a cotton ball with hydrogen peroxide and hold it on the spot for a couple of seconds.
Take your shower (warm water). I use a loofah with a handle plus Head and Shoulders when I shower. Wash your hair and rinse with the Head and Shoulders. Then use the loofah very lightly on your back to get rid of any dead skin. Take your Head and Shoulders and apply to the back. I leave it on for about 5 minutes, then rinse.
Dry off, by patting your back, not scrubbing it. I let it dry for 3 to 5 minutes while I brush my teeth, clean ears, etc.
Once I'm dry, I apply Crest regular toothpaste with cavity protection to each spot. A dab. I recommend this being a different tube than your normal toothpaste. Using your finger on the end of the tube then brushing your teeth or vice versa seems wrong. It should dry enough within 1 minute to put your shirt on.
Sleep with it on. As soon as you wake up, run your water as hot as it gets in the sink and soak a clean washcloth. Gently rub your back down to get rid of all of the toothpaste, let it dry for a minute, then re-apply toothpaste. This helps to clean your back again from possibly sweating or even bacteria in the sheets (we wash ours once a week). With the toothpaste on during the day, it helps with friction from clothes, drying the pimple, and seems to protect your skin. It also helped me from picking or touching it during the day.
When you do this routine, you'll find it's not a cure all formula. It's a practical way to start feeling comfortable again and it doesn't consume your day. I go through periods of not having any at all to the one or two here and there. As soon as I remotely feel one is going to pop up, I apply toothpaste and see results within 2 or 3 days. For scarring, I would sit in the backyard with by back to the sun for about 5 to 10 minutes once or twice a week.
In addition:
I shower at night or sometimes mid-day depending on when I get home or based what I've been doing...mowing, outdoor activities, etc. After I shower, I do the exact routine above. Also, if I go to the movies, travel, or sit for long periods of time with my back against something, I use a hot washcloth to clean my back. Hint: Keep a travel tube with you.
My theory for getting back acne isn't diet, but from the toxins that were in my body for so long. No different than it taking 10 years to clear your lungs after smoking cigarettes, I feel it's possible I've been dumping the crazy amount of built up chemicals through my skin for all those years of smoking as a detox. That's not to say every smoker that quits will have back acne, we know that's not true. And please don't start smoking again if this is the case for you. I'm just saying this could be particular to me.
If you're going through this crap experience and decide to try this routine, I would be very interested to see your results. From beginning to end, it took about 6 weeks to see a noticeable change. The quicker you can stop picking the faster you'll likely see results.
Best of Luck!