What's the best way to stop a baby who breastfeeds from bitting/playing with the nipple?

Unlatch your baby immediately and chide her in a firm tone. She will then learn that if she bites, she doesn't get milk. Sometimes, babies bite down because the letdown is too fast. It might help to express out some milk before nursing, or changing your position. I usually nurse my baby lying down. http://sg.theasianparent.com/breastfeeding-teething-and-biting-babies/