M.O.
Try a new toothbrush. My kids LOVE the ones with the lights in the handle. They know that the light goes off when it's time to stop...light on...keep going.
Also, let her brush, while you brush. Then tell her, when she's done, you get to brush her teeth to "check them". Then let her "check yours".
If this doesn't work - make a consequence for "not brushing" properly. No sweets, treats, etc.
Also, read the "Dora Goes to the Dentist" book. My girls learned about the dentist and cavities from that.
Finally, if you haven't already, take HER to the dentist - either your or your own. Our dentist counted our childrens teeth, gave them a chair ride and made it fun to learn about your teeth and how to keep them healthy. Eventually when they went to a pediatric dentist, it made sure they were coached on how to keep their teeth healthy. (Like if they're going to have sweets, give them all at once instead of spreading them out over hours (pottytime rewards) or days (Halloween).)
Hope this helps!
Sara