Cola, tea, smoke, acidic juices and highly pigmented food slosh past your teeth and your teeth tell the tale in the form of stains. Not that teeth were ever meant to be totally white as the natural colour of teeth is actually light yellow to light yellow-red.
Stains can be caused by antibiotics, by quirks in individual metabolism and sometimes by high fever. All these have to be fixed by a professional. But common stains can be washed away between professional cleanings:
Clean after every meal
Clean our teeth regularly and conscientiously.
Polish with baking soda
Mix baking soda with enough hydrogen peroxide to make a toothpaste and brush those stains away.
After every meal, rinse the food from our teeth.
An electric toothbrush will push more of the stain collecting plaque off our teeth.
Mouthwashes that have an antibacterial action will reduce stain-catching plaque
Do not use the super-whitening tooth polishes
Tooth polish is a quick fix, but it is like using an abrasive on a countertop. It takes off the stain but it wears off the enamel too. When our enamel gets thinner, more of the dentin shows through. As dentin is darker, so it looks like our tooth is stained.