Make her do something as the others said, and say eeeeeasssssyyyyy..do not let her have it until she takes it gently, plain and simple. She'll quickly go from a full bite to a gentle nibble. Hold a treat in your hand with your hand closed. Offer it to her, and if she goes after it, pull back immediately. She'll try different things to get it, and when she realizes the behavior you are after she'll catch on.