Riley had a really bad biting problem when she was a baby (not puppy nipping, but actual biting).. we just made a huge deal out of it, usually I would scream "OUCH!" and then I would bop her on the nose. That way she would know that she wasn't supposed to do it. Over time, I would just gasp really loudly whenever she bit me, and she would drop down on all fours because she knew she was going to get bopped on the nose. Now, whenever we're playing and she accidently nips me, she just automatically drops down to the ground and lays her head down, then licks me.