Are you strictly outdoor training or using piddle pads too?
Do you have just the 1 dog or do you still have the Rat Terrier too?
Have you had the Yorkie since she was a puppy, or is she a recent addition to your family?
Here is my blog about potty training:
Potty Training Small Dogs - Dog Life and Style
The basics are: limit roaming freedom in your home while training; supervise when possible; give ample opportunity to potty in the right place; praise and reward when she gets it right; don't get angry when she has accidents; be positive!