make sure you give them a little bit of salty food regularly.It helps make teeth strong(both in animals and humans)..
You can use a small toothbrush and scrub gently but it's easiest just to buy special treats (like Milkbones) that clean teeth as they chew on them.
I asked my vet the same question. It's good that you are asking because small dogs have a higher risk of developing gum disease. My vet told me to use a wash cloth and warm water. He said doggy toothpaste was not necessary and this method is just as good. Wrap the toothbrush around your forefinger and gently rub. It helps to start this sooner than later so they get used to it. My vet also told me to feed my dogs carrots for treats. They are healthier and they still get a sweet treat that helps to get rid of plaque build up. Of course, if your dog food is hard, then you are working towards healthier teeth as well. I was told the Milkbones are very fattening and you shouldn't give your dog more than 1 a day. I buy mine mini carrots and they love them! They can have as many of them as they want without worrying about weight gain.