The most effective way to get a puppy to stop nipping is to yell ouch loudly and turn around and ignore him for about 1/2 minute before playing with him again. By doing this you are teaching him what is an acceptable level of mouthing in order to keep the play going. Puppies love to play and start to learn what stops play as early as 4 weeks with their littermates. You are simulating this similar to what he would get in the pack when you stop the play because his bite was too hard. DO NOT hit him or startle him with a newspaper. Your goal is to raise a healthy secure pup who is confident and not to build a fear factor into his training.