lots of raw meaty bones, and if you still do need more cleaning, dissolve 1/2 teaspoon of ascorbic acid (vitamin C powder, but not buffered vitamin C - doesn't work) in a cup of filtered water and brush the teeth with that once a day until they are clean.
just like in people, good teeth in dogs are a genetic trait, so some dogs need more help keeping their teeth clean than others. the homeopathic remedy fragaria vesca (wild strawberry) in 6x dilution can be used to soften existing buildup on teeth by giving one pellet daily, and for maintenance it keeps plaque from forming on teeth, one pellet per week is often sufficient.
if you don't want to buy the ascorbic acid and the fragaria vesca, you can also add 10 pellets to half a cup of water, let sit for 5 minutes and then remove the pellets and ise the liquid to brush the teeth. again, once daily until the teeth are clean, and fromt here on out as needed.