Hi,
Brushing teeth always seems to be a battle in my house. I have a 5 year old and a 20 month old. With the 5 year old, I only brushed her teeth at night at first, as she was so difficult with it. She also would not take any form of fluoride supplement. She has had 7 cavities. After lengthy negotiations and pleading, she now takes a fluoride pill and brushes her teeth twice daily without much grief.
With my younger child, who is also very difficult, I make sure to brush at least twice a day. She brushes (or chews the brush) first and then I make the attempt at brushing. I don't always do a good job, but at least she is learning that this should be done on a regular basis. She takes fluoride without any problems.
I have tried many methods, like taking turns, having them brush my teeth, brushing away the "sugar bugs". Some toddlers like to make a game of it, so that may work for you. It doesn't work with my younger one.
Keep trying to brush her teeth. I would also recommend talking to the pediatrician for a fluoride supplement, if that is appropriate for where you live. Although I feel that my 5 year old's teeth problems are probably genetic, I think that the fluoride would have helped.
Good Luck!
Jen