Again from Dr. Dunbar, there are four stages of bite inhibition. The first two stages involve decreasing the force in the bites. The second two stages involve decreasing the frequency of the bites. The training *must* be done in that order. If you decrease the frequency first, the dog won't learn to soften his bite. The stages:
1. No painful bites. 90% of puppies will stop if you give a high-pitched squeal or yelp. If they stop, praise and reinforce by continuing the game. The other 10% and puppies who are tired or overstimulated will escalate their behavior instead of stopping. This requires you to confine the puppy or end the game. Remove all attention. It does *not* require any added aversive -- yelling, popping the nose or under the chin, shoving your hand down his throat, or spraying with water.
If you end the game, you need to be able to get away from the puppy with as little fuss or attention as possible. Even negative attention is attention. It's often helpful to have the puppy tethered, so you can simply move back out of his reach. Or, have him in a confined area and simply stand up and move past a boundary. Because the getting up and moving is tough to do at the instant the undesired behavior occurs, consider using a hand signal that will always mean "You're a jerk. Fun's over." Use it consistently when poor behavior occurs and you're going to withdraw attention.
I am well aware that puppy teeth hurt, and that this step can be overwhelming. Do it when you can, and at other times redirect, redirect, redirect. Puppy mouthing is a 100% natural dog behavior. It's not dominant. It's not meanness. It's a puppy being a puppy. When it's too much either redirect or end the game. Aversives are confusing, unfair, and unnecessary.
2. Eliminate all pressure. You want to gradually shape the dog to "gum you to death." Service dog trainers do this routinely, because service dogs often have to use their mouths to manipulate human limbs. Basically, you do this gradually. Set a limit of how hard the dog can bite. If he bites harder, yelp. Gradually set your limit for softer and softer bites. Remember to do this gradually. A big jump in criteria is confusing and frustrating to the dog.