Activated charcoal is apparently very good at removing teeth stains (I have yet to try it myself as I have to go and buy some). You make it into a paste and then brush it on. Whiten Your Teeth Naturally With No Dangerous Chemicals
Oil pulling with CO has dramatically whitened my teeth. I also brush with a paste of CO, baking soda, and peppermint oil. The trick is to brush much more gently with the baking soda. Flossing is also important!