There's a grooming section here:
Yorkie Grooming Talk - YorkieTalk.com Forums - Yorkshire Terrier Community
It's best to brush out all the tangles before a bath, otherwise the water makes it tighter. If it's a stubborn mat, I usu snip it once w/a blunt nosed scissor & work it out w/a comb. The high rubbing areas (arm pits) should be shaved to avoid mats, since that area can be sensitive for them.
I always use a conditioner after a shampoo & spray their coat w/a conditioning mist before brushing them to avoid hair breakage.