When my DS bit, I pushed his face into my breast which made him let go, told him he must be gentle, and ended the nursing session for a moment. If he wanted to try again, I let him. It didn't take very long for him to realize that the end result of biting was the end of the nursing session, and he stopped. He tried it again after he got teeth, I used the same tactic, and it was over with even quicker. And just so you know, it wasn't exactly a fun time with teeth, but it wasn't nearly as bad as I thought it would be, especially since we nipped it in the bud when he was only giving tentative, "testing the waters" sort of bites, versus the bearing down, full force bites I was getting when he was toothless. The toothless bites were way worse.
Also, one thing I noticed about my DS was that he was way more likely to bite when he was full or getting bored, so paying attention to the behaviors he displayed shortly before a bite went a long way in preventing it to begin with. When I saw those behaviors, I knew it was time to go ahead and end the nursing session and move onto something else. I avoided the bites, and he was fine with it, because he really was done nursing anyways.