Your vet probably won't agree, but one of the best things for reducing plaque build up is a raw chicken wing every now and again.
Honey has been completely raw fed from a young age, and has pretty good teeth... but my Dad has noticed the difference in his older Collie since moving onto raw food...
I think if you are sticking with a commercial food, a chicken wing once or twice a week is generally enough to have a good impact on teeth... (raw chicken bones are fine for dogs, despite what you may hear... it's cooked bones (in general, but especially cooked chicken bones) that cause the problems!)
If you don't want to try this, I believe some people have had good results from a product called Plaque Off...