Exfoliating helps a lot so you can make a paste out of corn meal it's inexpensive and I love that. Massage vigorously, first with your hands then with a loofah.
Exfoliate on a regularly basis to help you get rid of unwanted skin cells that might exacerbate back acne. And comparing the skin cells on your back to your skin on your face,the skin cells is generally massive and thicker.
This is just for general acne, if you have severe then consult a dermatologist for medicines such as Accutane.