Acne is due to a hormonal imbalance, you could try two things, one being adaptogenic herbs which balance the hormones but also increase the bodies resistance to stress. They have mountains of other benefits also.
Or you can use bitter herbs on the back of the tongue which cleanse the liver. The liver being the master controller of hormones.
Some people claim that toothpaste is a natural home remedy for acne because it can dry out pimples, but the the though of smearing toothpaste all over your back might be a bit much. Wheatgrass juice is another good idea (I am a huge fan of wheatgrass in case you didn't know). You *can* apply it as a paste, but drinking it regularly also helps clear up the skin.