I just started a thread not long ago asking the same thing. You can view it here
http://www.yorkietalk.com/forums/yor...sit-still.html
Lots of good advice in there. Basically you just have to hold them down and do it. Sometimes it takes 2 people if they are overly aggressive, I learned this at the groomers today as I started apprenticing today
Because it's better to have someone help hold them down than you try to do both and then cut too low on accident and get the quick. This will really make them hate it more!