Make sure you get a copy of the contract and health guarantee now, before you pick the puppy up, so you have a chance to look it over.
As Marcella said, make sure the breeder is able to show you documentation that both parents have been checked for liver disease and luxating patellas. Yorkies are 36 times more likely to have a liver shunt than all other breeds combined. The Orthopedic Foundation of America ranks Yorkies #4 in cases of luxating patellas. Treatment/surgery for these conditions runs into the thousands so you want to make sure your puppy will be free of these inherited conditions.
Ask to see the pedigrees of both parents now, too. You want to see champions in the first three generations to make sure the breeder is breeding quality dogs. You want to make sure that a puppy you buy will grow up to look and act like a Yorkie is supposed to.
Look at the YTCA Code of Conducts and Code of Ethics. It tells you what a responsible Yorkie breeder should do. If a breeder violates any of the breed club's rules, find another breeder.
http://www.ytca.org/frame_index2.html