I am definitely not an expert on Yorkies but I know using an Xpen has really trained Kiki on where to potty. From the day we brought her home I put her pee pads in the Xpen and she would stay in there until she pottied and then we praised and praised her with treats and hugs. We would let Kiki out after each potty but we very quickly learned her signs that she had to go again - circling and sniffing on the carpeted area. We watched her like a hawk and put her back in her Xpen until she pottied. She is year old now and I honestly think she has had only a handful of accidents on the floor and that was when she was only a few months old.
Good luck!
Anna
http://www.dogster.com/dogs/511397