I use those little child toothbrushes, usually on the package it says something like "kids first toothbrush" or something. I got a two pack at the dollar store. The bristle area is like a centimeter long. Make sure to get one where the handle near the bristles is very thin, and long enough to maneuver comfortably. There's also a pet tooth brush that fits over your finger. Its a rubber sheath that you stick on your finger, and it has tiny rubber bristles. This is good for cleaning the gums gently. Cats usually actually chew on it a little, so they kind of clean their own teeth with it. Another option, though I've never tried it, is dental gels and sprays. You spray it in their mouth or stick a glob of it in there when they yawn and they chew and lick around their teeth, spreading it around on there own... but if you do use the spray or gel, you should still do a bit of brushing. Think of the spray/gel vs brushing like hand sanitizer vs washing your hands; while hand sanitizers kill the bacteria, it doesn't actually remove dirt like washing your hands.