Some of my favorite products:
CET vanilla mint toothpaste
Petzlife gel
Petsmile toothpaste
HealthyMouth (they have a water additive and a gel)
^ these 2 are VOHC approved, which basically just means... they're actually *proven* to work.
Here's the full list:
http://www.vohc.org/VOHCAcceptedProductsTable_Dogs.pdf
I give Purina DentaChews as a treat as well.