It's important to know the difference between biting, nipping, or mouthing that is employed during play, and when it's more serious. While a dog or puppy may growl and bark during play, as long as there is plenty of loose and wiggly body movements, you are probably still in the play zone. Stiff and rigid dog bodies usually indicate that events have taken a dark turn. If you ever feel unsafe or threatened, simply end the game. Wait until the dog has settled or walks away before you leave the area. Seek the services of a qualified dog behavior specialist, an applied animal behaviorist, or a veterinary behaviorist for further advice.
Play-biting is recognized as a non-aggressive act. It can happen during vigorous play interaction with another animal or with a person. There is no reason to be alarmed by this behavior. Still, it can hurt and it should be modified.
Puppies (and some adult dogs) need to be taught how to control their mouths and wild play behavior. Since human beings are not covered in thick skin and fur like other dogs, dog teeth can make direct contact with sensitive skin. When this happens, say "OUCH!" in a sharp, short tone. Do not move away from pup as this may only serve to cause your dog to chase you. Do not re-engage your pup in play until your dog has sufficiently settled. Soon pup will learn that rough play ends his fun.
It is also recommended to use a toy to play with your pup. Direct your puppy's mouth to the toy and he/she will learn to grab a toy when feeling frisky.
Most dogs are pretty good at letting a pup or other adult dog know when play has gotten out of line. But don't wait until it breaks out in melee. Interrupt your dog when you notice that he/she is thinking of getting rowdy. Redirect your dog's play to a toy or ask him/her to settle. Allow your dog to join back in the fun after calming.