I swear by these things:
http://www.cetdental.com/good_to_chew.asp
but they can be a little pricey, and you have to get them at a vet. the CET chews. I buy big bags and cut them smaller- comes out a little cheaper.
Many times, esp in small dogs, teeth don't align properly, and so things when eaten don't move in the mouth like they would a normal dog, and some dogs simply have VERY chronic, bad plaque. Aside from dentals, I'm not sure what all you can do- perhaps someone else will have some suggestions!