Only thing you can do to remove the issue is to stop taking steroids.
Otherwise if you get acne on your back then its something you need to live with or you need to look at lower doses or other compounds that dont cause the bacne.
Other things as suggested are bathe 2-3 times a day, use a good brush and get a body scrub with acne medication in it or even use some baking soda as its a good exfoliator.
Like for me I get minor acne if I keep my test dose in the 250-300mg/week range. When I go to 500-600mg/week all hell breaks loose.
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Forgot to mention I would not recommend going for an acne medication like Accutane.