How old is your dog? What breed?
Brushing teeth is THE best thing to do. Use a toothpaste specifically for dogs, human toothpastes will make them sick.
You can get oral rinses for dogs, but they are specially formulated for dogs. DO NOT use Listerine or Scope or any of that crap. Other than the alcohol, a lot of human dental products also contain Xylitol, which is extremely toxic to dogs. Even a small amount can cause serious problems. Teeth need to be brushed at least once a day, twice a day is better. Brushing just once in a while won't do much.
You can also give dental chews. CET Hextra chews, as mentioned above, are really good. There's also Greenies, regular rawhides, or good old raw meaty bones.
Soft (canned) foods can contribute to dental disease. So can extruded kibble (most kibble is extruded, unless it specifically says it's baked). Baked kibbles have more of a scraping action than extruded kibbles, which can help clean the teeth.
Some dogs will let you scale the teeth, too, to get buildup off. Both my dogs let me hand-scale their teeth, and I do so when I notice some buildup. But the vast majority of dogs will have nothing to do with this.
Your vet may also have, or be able to get, Oravet. It's a sealant that you put on the teeth once a week. We don't have many people who use it, but the few who do have dentals less often than those who don't use it.
And then there's chance, genetics, and whatever else. Max, my parents' 12.5 year old GSD has never needed a dental cleaning. He's not a chewer, he's never had the best foods, but his teeth are still great. Some small breeds seem to need a dental every 6 to 12 months.
