Heat and friction are the main components for back acne, also sometimes referred to as athletic acne. You will see this form of acne is common among athletes. Select a loose fitting cotton t-shirt. Be sure to apply benzoyl peroxide or witch hazel to your back before performing any exercises or activities where the probability for sweating is increased. Benzoyl peroxide will kill any bacteria that may accumulate on your back as you sweat.
Witch hazel is good for drying up oil and toning the skin.
Also be sure to wash your back immediately after an exercise session. Don't forget to change out of your sweaty shirt into a new fresh one. You should probably change your shirt twice a day whether you exercise or not to avoid bacteria buildup that can further aggravate your acne.