There are two things that I have tried, that seem to work for my daughter:
1. Sit her facing you and let her brush your teeth while you brush hers. You'll find that when you open up and say "ahhh" or "cheese" she will start to mirror you. She will be concentrating on brushing your teeth and it will take the focus off of hers.
2. Now that my daughter is a little older (but still a little reluctant at times) I let her look at herself in the mirror while I brush her teeth. I encourage her to make silly faces. She likes that!
I also have found that when I immediately start to praise my daughter "WOW! You are doing such a great job tonight with your teeth!" "You're the best!"... she opens her mouth wider and will tolerate a longer brushing.
Hope that helps...good luck!