Biting/mouthing

Heidi is approximatley 12 weeks old. She's very smart and learns quickly. One problem we have with her, however, is that she like to mouth/play bite with us. It doesn't feel like play biting, but I know it is.How do I get her to break this habit. At her age, it is a bit of a problem due to sharp little puppy teeth, that's why I wish to end this NOW, before she is big and can do real harm. As well, when I repremant her, she talks back! Sassy little thing she is.  I would like to break her of this too., Thank-you,   Marie Lucas (Heidi's mom)