Capital News 9 has published an easy recipe for dog biscuits that freshen your dog's breath:

Ingredients

2 cups wheat flower
1 egg
3 tablespoons safflower oil
½ cup freshly chopped mint
½ cup chopped parsley
½ cup water

Method

Preheat oven to 375 degrees. Mix the egg with the water and oil in a large bowl. Then, add the mint and parsley to the bowl and stir everything together. Slowly add in the flower and make sure you stir it as you do.

The mixture should get solid, so you can begin to ball up your treats for baking. If the mixture is too solid then add a splash of water and if it is too wet then add a dash more of flower until you get the proper consistency for baking. For bigger dogs a good 1½ to two inch ball will do on a cookie sheet, but if you have a smaller dog it would be best to do one inch balls on the cookie sheet.

Cook in oven for 15 minutes. The time will vary depending on the size. If the treats do not seem fully cooked then let them bake for just a bit longer. Take them out of the oven and let them cool for 30 minutes.

There is also a pretty good video that discusses ways to keep your dog's teeth clean, including a demonstration of how brush a dog's teeth.