I occasionally brush my cats' teeth ( I know I should do it regularly). They hate it, but are surprisingly good about it. I use a baby toothbrush. First I used one of those soft ones that you put over your finger, but it didn't seem to do much. So I bought a toodler size one with proper bristles, still very soft though, so it won't do damage.
I use Hartz beef flavour toothpaste which I just buy from the supermarket. I have a feeling that there's probably a better quality product out there somewhere!