I have suffered from pimples on my back and buttocks for most of my adult life. What I've found to be very helpful is to use a clay mask regularly - 1 to 2 times a week on the problem areas. I've even left it on over night.
Mint julep mask is very good for this.
Another tip: ladies, change your bra daily, I noticed I was getting breakouts in the area where my bra straps were - on the shoulder blades. I believe the sweat and dirt buildup on the straps contributes to the problem.
Make sure you are not constipated either, it will exacerbate the problem.