If you watch dogs play together, they often mouth each other in a sort of mock bite. Many dogs play with people in the same way — by mouthing our hands or other body parts. Though mouthing is not biting, it can become too aggressive to be acceptable.
To discourage mouthing, always use a toy to play with your dog. If you inadvertently become the toy, say “Ouch!” in a loud, surprised tone and remove your hand from the dog’s mouth. Wait just one second, then offer your hand for licking. If the dog mouths your hand again, repeat the steps above until the mouthing stops. When she does not mouth your hand, praise her and introduce a toy. You can then throw the toy and say “get the toy” to start a game of fetch.
For her to get the idea that mouthing is not acceptable, your dog will need lots of practice with the above technique. As with other training techniques, consistency is key, so try to make sure that anyone who plays with your dog knows how to discourage mouthing.