Keep trying! This is SOOOOOO important! I work for a dental office and have seen kids come in with 12 cavities (or worse) by the age of 4! This is actually something that is still somewhat of a struggle with my 3 year old, although much better than it has been. A couple of things that worked for us.
We called them "cookie bugs" and even gave them names such as Grandma, Grandpa, sister, etc. I'd offer to let her get the big ones and then I'd go in for the "little ones" and brush her whole mouth thoroughly.
Sing a song when you brush. Make up words to a familiar tune about brushing teeth.
When brushing, open your own mouth wide. Kids (and adults for that matter) have a natural tendancy to mimic body language. Try this, it really works.
Lastly, once the task is completed, TONS of praise. We tell our daughter "WOW, you have the whitest teeth around" then we flinch and cover our eyes and say, "Your teeth are soooooo bright that we need sunglasses. Shut them off!" and make a silly deal about it. If she doesn't brush as well, we don't make a big deal about it. She wants us to so she wants to brush thoroughly. And sometimes she refuses to eat anything after she's brushed so she doesn't get them dirty!
Also, brush for 2 minutes if possible and get into this lifelong habit and try to get all surfaces of the teeth. It is also never too early to floss. MANY MANY kids who come in to our office with decay have cavities not only on the biting surfaces, but between their teeth.
It is unbelieveably expensive to fill teeth. And people think, "oh they are only baby teeth". But decay is bacteria that gets into your system and can affect your whole body. How baby teeth are taken care of can have an effect on how permanant teeth form.
Definitely special kids toothpaste and a brush as well.