In my opinion, potty training is definitely the number one issue with yorkies. My little boy is only 6 1/2 months, so I can really only give you a partial picture with my experience.
First off, I entered my relationship with him not expecting him to make great strides in this area until around 1. I just did not want to get frustrated with him at any time over potty training, so I set what I thought was a reasonable time table.
Potty training takes constant attention and being able to anticipate the dog's needs. That was key for me. Also, it's gotten better with time because his bladder has been growing and he's starting to get bladder control.
At 5 months I noticed that he was really getting the hang of it, and understood that he was supposed to go outside - he would go and jump up at the door. However, he still does have accidents. He can go a week without one, but still he has them.
Good luck, I'm sure you'll get some better responses too..
