Older nursing babies often know when they are biting down, but if this is a behavior you have ignored until now it will be harder to correct. Start by having your milk "ready" by hand expressing for a few minutes before you nurse. Then, when you start the latch, place your nipple deep in the babies mouth, avoiding the tender area next to the end of the nipple. Once latched, if the baby bites down, scoop you nipple out and say "no bite". Your baby may get upset that you are setting limits, but if you do not do so know, you may end up stopping breastfeeding. If this does not work, or you want other options, try a good Lactation Consultant, breastfeeding clinic, or La Leche group
Good luck!