I was 6 when we got my first yorkie, Kiwi, so I wasn't in charge of training. My mom did a great job training her, though. Whenever she would bark (and it wasn't at the door to be let out, or when the doorbell rang or if someone knocked or at a cat, etc...) she would first give a warning by saying sternly, 'No Barking!' Then, if the barking continued, she would give her second warning a little more sternly 'NO BARKING!...Where is that mouth?' and if she barked again my mom would go over to Kiwi, and put her thumb and index finger around her snout and firmly (just tight enough to hold her mouth closed when she tried to get out of the hold, but not to hurt her) and would repeat 'No Barking' again. After a few weeks of that Kiwi learned not to bark. On occasion when she did bark at an inappropriate time all we had to say was 'no barking...where's that mouth?' and she stopped right away. She knew. For 16 years after that she was good as gold!
I plan to try the same method with my Strawberry if she starts to bark too much. As of now I am not worried. I have not heard anything that I would consider a bark out of her. She growls to let me know when she wants something and a few times at the huge cat outside. Hahaha! It ran away! It is a huge, 30 pound cat and it ran from my 2 pound puppy! Hahaha!