An acquaintance of mine, she used to have bad skin acne on her back. She says that though tea tree oil and such helped, the thing that helped her the most is frequent sunbathing. I imagine it's thanks to the UV-light (which kills stubborn bacteria deaddead). She isn't horribly tanned, just a light tan, but apparently it works wonders. Beware of too much of that, apparently frequently going to a tanning-salon gives you a 50% chance of skincancer.