When Snoopy was little, we gave him a child sized toothbrush to play with, so he'd get used to it. He loved to chew on it. Still doesn't like his teeth brushed, though.
Around here, it's a 2-man operation. One of us pries his mouth open--with thumb and forefinger, from the top. The other tries to get in with a toothbrush. I hope what we miss this time we'll get next time!
Hope this helps!