Puppies usually nip or bite in the excitement of play or for attention, but never let him get away with it. Your puppy does not mean any harm with a friendly nip, but he also does not know that his friendly nip can hurt. The dog who nips is a nuisance. If you have children in the house, certainly you must train the puppy or dog not to nip even when the children provoke him and he nips in self-defense. The best time for this training is during puppyhood.
Mouthness, nipping and biting are normal actions for puppies. His mother and littermates were his first teachers to tell him not to do this. Now that the puppy is living in your pack, you replace the interaction that the puppy would have had with the other puppies and dog. You must step in and continue with bite-inhibition training in a manner that is perceived as normal by your puppy.
Excessive nipping very often communicate urgency: the puppy needs to go outside, he is tired, or he has too much energy and is very excited. All these needs are often communicated through unrelenting, hard bites. Before you start correcting this, remember that when he displays these behaviors, he is trying to tell you something. Disciplining him only creates confrontation, aggravates your relationship and undermines the trust he has in you.
As puppies start to teeth, they feel the need to sink their teeth into anything, including your fingers, arms, hair and toes. If your puppy shows the least sign of biting or closing down too hard playing, discourage this tendency immediately with a loud and firm "No!", then give him a chew toy. Don't pull your hand away or push the puppy, as this only encourages rougher play.
With a puppy that already shows a bit of aggression, don't play tug-of-war with him. It tends to make him more competitive. The best way to break a puppy from nipping is to substitute play for his habit. When he comes toward you looking as if he might nip, reach out to pet and give him a toy. If he persists, take him by the collar, scold him, looking straight in the eyes for several seconds while you repeat the command. Release him and tell him quietly that he is a good puppy. Reach out to touch him, and if he starts nipping at you again, repeat the correction.
Another effective correction is to isolate the puppy - totally ignore him as soon as you say "No" or "No Bite". Fold your arms and don't even look at him. After a couple of minutes, give your puppy another chance to play with you. Repeat the isolation routine as often as necessary until he goes easy on his nipping.
If your puppy is nippy and wild when the kids are around, place him on a drag lead (a short 4-6 feet light-weight nylon lead that stays on him in the house). Use it for quick corrections, but don't yell at the puppy as he views this as confrontational.
An older puppy that starts this unpleasant habit may take a little more firm discipline. When he nips, wrap your hand around his muzzle and squeeze gently, saying harshly "No". Then continue to play with the puppy, so that he will learn that it is not playing that you disapprove of, but the nipping. If gentle squeezing produces poor results and your puppy continues to mouth and nip your hands, quickly clasp your hand around his muzzle and shake it as you say "No!". He should whine in displeasure. Then open your palm for him to lick. If he does so, praise him - licking is OK. If he tries to nip at you again, repeat the correction.
Occasionally, a puppy will bite not in play but out of fear or dominance aggression. This behavior demands a very stern "No!" in your voice. The swiftness of correction is extremely important. Do not talk with the puppy after this correction for several minutes. John Ross offers an interesting method to discourage mouthing and nipping by using "dog talk": a guttural sound imitating mother dog's growl. It is low in tone and comes from deep of the throat : "Nhaa!" Most puppies instantly understand what this means and they stop whatever they are doing. You should growl every time your puppy nips or bites you. The better you become at growling in a timely and consistent manner, the quicker your puppy will learn. When he stops nipping and starts licking your hand, praise him.