We get goofy with our kids, and say stuff like, "uh-oh, I see a camel in there, let me get that stinky camel! - now there's an elephant way in the back, aaaah, quick, let's get him before he escapes! - aww, look at that cute little baby zebra, let's clean him up for his mommy so he doesn't get mistaken for a water buffalo!" Stuff like that, the goofier the better, always works to get them to let us brush their teeth without a fight. They usually just start giggling so much that we are able to get in their mouths and brush. The trick is to keep finding other animals that need to be either removed or taken care of so the fun continues the whole time.
We've been doing that for years, and it still works!