For young puppies, nipping, biting and "mouthing" are normal behaviors and rarely cause serious harm. But it's important to stop these behaviors before your dog reaches adulthood, when a bite could inflict serious injury.
Begin your training early - when your puppy is 6 weeks old, if possible.
Teach your puppy that biting hurts you. When he bites, give a sharp "no!" or "ouch!" - or even a yelp like a dog would make. This is the same reaction that your puppy would get from another puppy if he bit too hard during play. It teaches him that he's been too rough, and the odds are that he'll be more gentle next time.
Remove your hand slowly - don't jerk it away - and give your puppy a toy that he can chew on without damage.
If your puppy ignores your reaction and bites again, repeat your "no!," "ouch!" or yelp, and leave the room for a few minutes. Let your puppy know that when he bites, he will lose his playmate. This, too, mirrors the reaction he would get from another puppy if he played too roughly.
Be persistent in your training. As your puppy's behavior improves and his bites are softer, continue to yelp or give a sharp "no!" or "ouch!" whenever he puts his teeth on you. Make it clear that all bites are painful and unacceptable to humans.
How to Stop Your Puppy From Biting Provided by eHow.com