I use a technique called Oil Pulling.
You basically put a tablespoon of oil (olive oil/coconut oil is recommended) in your mouth and swish it around for about 20 minutes.
It supposedly sucks out toxins from your mouth and your body.
I think it whitened my teeth a bit, though I am not sure. I do use this technique for only half a year though, furthermore I've been a bit inconsistent at times.
My dentist seemed to notice it though, when I visited my dentist after starting to use oil pulling he actually didnt do anything but looking into my mouth and saying that everything was fine. I never had that before.
Most people on the internet claim that it had whitened their teeth a lot, and since it doesnt have any negative side-effects you should give it a shot!