Lot's of Yorkietalk members use Petzlife gel. It removes tartar, and says if it's really bad, it takes about three months of daily use. I use the spray about 2-3 times a week and the gel during his weekly groom session. They recommend regular tooth brushing as the Petzlife softens the tartar, and the brush scrapes it away. You can get spray or gel, the Petzlife site gives more information, but you can purchase it for less if you do a google.
PetzLife Products Inc - Products For a Healthier Pet