Same problem here. My little monster won't let me brush him without attacking the brush.
The last time I took him to the groomer I asked for a trim and because of all the mats she wound up giving him a total buzz cut!! I have since been trying to comb him everyday. I distract him with a chew toy while I brush him quickly. For those of us with cottony yorkies I don't think you can totally get rid of all the mats. They are bound to happen no matter how much on top of them you are.