When my flat-coated retriever, Lady, was around 6 weeks old she started mouthing and it would really hurt since she had razor-type little teeth. I read an article on the web that said when teeth touch skin to scream "ouch", pick her up and put her in the crate for 2 minutes and walk away. It took 6x for her to stop, but she really got the picture. I use that approach when she misbehaves and since Lady wants to please me, when she does something wrong, she catches on pretty quickly.