Eat more fibre, less sugar.
Drink lots of water.
You can use super fine grit charcoal mixed with coconut oil to polish teeth but this will wear them down and destroy the enamel if you are not careful.
Oiling pulling after brushing teeth will also help; Swish coconut oil/tallow or some other minimally processed high saturated fat type oil around in your mouth and hold it for a few minutes before spitting it out, then rinse with water.

Cut back on coffee and cigarettes, if you smoke or drink a lot of coffee, trying to keep them perfectly white is going to do more damage than its worth.