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

When my baby bites, I will unlatch him by gently pushing his chin downward. I will then give a sad face and say "no, no, no, mommy pain pain". After doing this a few times, he seems to understand what I mean and he never do it again. Babies are learning too. We have to slowly teach them what is right and wrong.