I've never been able to clean Max's teeth, and consequently his are not great (also due to his FIV I reckon).
If your furries eat wet food, I'd recommend trying Plaque Off, its dried sea kelp (doesn't taste or smell of anything particularly), you sprinkle that on their food and its supposed to help the plaque forming on their teeth.
Also you can get the cat toothpaste, Logic Gel which is also supposed to help keep teeth clean, even without brushing.
My local vets now stock both of these items and they're pretty inexpensive too.
Hope this helps.