I have heard that you should brush your teeth and make it look like heaps of fun ... pull faces, spew the foam in the sink/bathtub ect. and let her mimick you. Young children like to mimick parents and other children, that is how they learn. Then ask her if she would also like to have fun with you. I have also heard that this is more sucessfull when she is in the bath, that way she can brush away without you having to wipe up afterwards Maybe buy her a funky toothbrush and special DD toothpaste (pink tube, or funny pictures)
I don't have firsthand experience, but Hope this helps!