Natural teeth whitening?

I heard that if you brush your teeth with baking soda, your teeth would become pearly white. Then later on, I read that if you collect ash (after a barbecue etc) and use that to brush your teeth, it'll be white too. There's been no chance for me to try the ash one but the baking soda one does not work at all. Those whitening toothpaste doesn't work either. Any suggestions for natural ways? (Using natural ingredients)