First you need to realize that biting is a totally natural behavior for dogs, both wild and domestic. Dogs bite each other in play and in warning. Very rarely do they bite each other in order to do serious damage. As pups, they learn the importance of bite inhibition by playing with each other. When a pup bites a littermate too hard, the victim yelps loudly and may refuse to play for a while. Thus the biter learns that the fun of play ends when he bites too hard. Usually over the course of the first 5 months of his life he learns to control the strength of his bite. If he doesn't have this opportunity, it is much more difficult for him to learn to use his mouth gently later in life. This is even more common in pups who were not properly socialized during the first 5 months of their lives in addition to being taken from littermates to soon.

My suggestion is imitate the puppy's littermates. When a pup bites hard, say OUCH or NO BITE in a loud, high pitched tone and remove yourself from the pup's reach for a few minutes. Then return to puppy play. Each time the pup bites too hard, repeat the lesson. After several repetitions the pup's bite will begin to soften. You can then repeat the lesson at gradually decreasing levels of bite intensity until the pup learns not to bite at all.

At the same time that you are softening the puppy's bite, you can also direct the puppy's biting toward acceptable chew items.

Another thing to note: Unless you are trying to build self confidence in a shy, nonaggressive dog, avoid wrestling and tug of war games. They tend to incite competitive struggles that your dog needs to settle with his teeth and hard mouth!