As usual, Jackie brings up an excellent point. A sleepy dog (or puppy) is a much more cooperative subject! Even though Lady is an adult and very used to all the fussing, I still usually brush her in the evening, in front of television, when she is sleepy.
Treats are also a great incentive for being good. Because of her diabetes, Lady has to get insulin shots twice a day, plus I check her blood sugar myself by taking a small drop of blood from the inside of her lip. Obviously, she has to be a very good and cooperative patient for all this. I always keep a little treat next to me and she gets it IMMEDIATELY after her shot or poke - as fast as I can get it to her so she connects the two. She sits as still as a statue for both, believe it or not.