Here's a thread that will tell you everything you want to know about their little butts.
http://www.yorkietalk.com/forums/yor...-grooming.html
For their ears, you shave the top third front and back. Ear hair growing in canal can be easily plucked with fingers. Some people use special ear powder so the hair isn't slippery. We have a wonderful grooming forum, that has some great threads, and stickies on grooming your Yorkie.
Yorkie Grooming Talk - YorkieTalk.com Forums - Yorkshire Terrier Community