Training a Puppy Not to Bite?

My three month old Pit puppy bites. I've had her since she was 3 weeks old. She has a bad habit of snapping at people when they are playing with her. I've tried everything from walking away, to saying no in a firm voice, and yelping when she bites. Nothing works. She's biting my nephew and my little sister. I don't know what else to do.

By Clarissa C