Candy is very right on this one. Presley my lab was a big on biting and chewing on us. We would yelp like she would if she got hurt.. It didn't take long about 5 times and she stopped for a good long while. She started again about 2 months ago and we yelped good and hard and she stopped again. I read in a book to keep doing it and be more dramatic about it (make it sound like he is really hurting you badly if need be).
I found with Presley though i would do it when she would nip and stuff but when she really hurt me i was too shocked to yelp lol.... I am glad she got the idea though.