First of all, like someone said you shoulden't ever smack a puppy and secondly holding the mouth shut might work for some, but it makes alot of puppies more aggressive and even more determined to bite you.
Since you are his favorite chew toy and companion, simply ignore him when he bites you. Everytime he bites you, say "ouch that hurt" or even say "yipe" (this sounds like a puppy yelping" and walk away. The yelping like a hurt puppy works wonders. He will learn that if he bites you, he doesn't get to play with you or be with you and that if he wants to play or be with you, he has to control his biting.
Have you ever watched a group of puppies playing?? When one of them is being too rough to them, the puppy will yelp and the other puppy will back off. They soon learn that they have to control their biting in order to be able to play with the puppies and know when they are being too rough. It is very important to teach them how hard to bite. Even if you can barely feel, react the same way as if he bit you really hard. Make a huge deal out of it, just like a wounded puppy would yelp and back away. This teaches them to have a soft mouth and is called bite inhibition. This is a very important lesson and has been known to save a dog's life. You have to understand that dogs think we are dogs as well and you need to speak their language to get them to understand what we want them to.
For example, you have a fully grown dog that was not taught to have a soft mouth but doesn't really bite you. Then one day someone steps on his foot, trips over him, hurts them brushing them, the dog's first response is to bite in reaction to the pain. Since the dog doesn't know the strength of his bite, he could end up hurting you really bad.
On the other hand, when a dog is taught bite inhibition, to have a soft mouth, then one day when you do hurt him unintentionally he most likely will react by biting but since he knows the strength of his bite, his bite will not be nearly as strong or hurt as much.
Even though yorkies are small, their bite can still do alot of damage.
Here is a short true example. An owner had a dog who was a great dog, most loving dog. Their child walks into the room and not seeing the dog steps on the tail. The dog reacts by biting and since he wasn't taught to have a soft mouth he hurt the child really bad putting him in the hospital.
The same thing happened to another kid but his dog was taught proper bite inhibition so when the dog bit him, it didn't even break the skin.
I have done this with all my dogs when they were playful biting puppies and they all learned quickly not to bite and if they were to bite, to control their bite by having a soft mouth.
I hope this helps you. I have not yet had it fail on me.
