Is your dog a puppy? Is it playful biting? What we do at the shelter when a puppy starts mouthing and biting us, we say "OUCH!!!" in a loud, high pitched voice. This gets most of them to back off. The ones who don't back off and usually the really dominant ones who require extensive training. The important thing is to make your voice really loud when you say ouch. Try this and see if it works.