When Lu had his scale and polish the vets recommended Logic gel, ideally you squirt it onto the gums but this would upset them so Lu licks it from the tube and Riley has a bit squirted on his dinner (he won't lick it from the tube). I have also used plaque off (a powder you sprinkle on the food) in the past. I am terrible at remembering to use both these so really must start again.
Whiskas do a treat called dentabits which my two love so worth a try too.