You don't! The bigger deal you make of it the more she will do it. I've been encouraging my 5 1/2 year old to stop for a few years now and she won't. Her father does it, his siblings do it, his mother did it and so on. I think it's just a trait that they will hopefully outgrow. In the meantime just make sure that she keeps her hands really clean and discourage her from doing it at school or in front of you.
You could tell her that if she goes for a few weeks without biting them you will reward her with something exciting, but I wouldn't count on it. Sorry, I don't mean to sound negative but if she is really a biter you're fighting a losing battle.