Puppy biting is a common problem among Labrador retriever puppies because labs are bred to be comfortable with objects in their mouth. Labs are hunting dogs, bred for finding birds and carrying them back to their owners.
Getting the Family on Board
When beginning a program to stop lab biting, it's important that everyone in your family react the same way to the nipping. If some family members allow it and some don't, the program has little chance for success. Sit down with everyone in the family, agree on the rules and game plan and stick with it. Consistency is the only way to reduce the nipping.
In litters, when a lab bites too hard, his litter mates don't respond by biting him back harder. They walk away, isolating him from the group. Instead of punishing, which may not be effective since you're still playing with him, agree to isolate the puppy by banding together and reacting the same way.
Redirect to Desirable Objects
First of all, it's important to teach your lab what is OK to chew on and carry in his mouth. Since this is a natural instinct, you need to provide your lab with appropriate toys to play with. Since he may also be teething, provide him with bones that can relieve the pain of incoming teeth.
Teach him that these toys are the objects on which he's allowed to chew by redirecting him each time he puts his teeth on you or something else you don't want him chewing on. Make those toys fun by running around with them, dragging them across the ground or playing fetch or tug with him.
Yelling "Ouch"
Once you've taught your puppy what he can chew on, stop tolerating any teeth on anyone in the family. As soon as your puppy puts his teeth on you, yell "ouch" in a loud, upset voice, even if the nip didn't hurt at all. For this to be effective, all rough housing that allows teeth on humans should be stopped during training.
The sound of your voice should startle your dog and cause him to look up at you. Praise when he stops nipping and redirect to a toy. Play a game with him to reinforce that he should have his teeth on the toy, not you.
Storm Out
If your puppy continues to nip after you have yelled "ouch" and redirected with a toy, yell "ouch" again and follow that immediately with storming out of the room. This should be dramatic. Labs don't respond to subtlety.
Stomp from the room angrily as if your puppy did the worst thing he has ever done. Slam a door. If there is more than one person in the room, everyone should leave.
Return in a few minutes and give your dog a second chance. If he licks, praise. Redirect with a toy and repeat until your puppy isn't nipping.
Timeouts
If you don't want to leave the room, keep a leash on your puppy when you are home to supervise. As soon as he starts nipping, or if he doesn't stop as you storm from the room, walk him calmly to a timeout spot such as a crate or bathroom and leave him there until he calms down. Once released, offer him a toy and try again. Repeat every time your puppy nips.
If you are consistent with either method, your puppy will learn that teeth on humans are never allowed and will begin to play by your rules.