The best way to whiten your teeth naturally, however not always the easiest, is to simply brush your teeth after eating or drinking something. This takes a lot of persistence and can even be kind of difficult depending on where you are at the time of eating (such as work or school).
Avoiding smoking cigarettes, drinking too much coffee and/or soda, improving your oral hygiene overall, and eating a healthy diet also help prevent yellow teeth. If you do regularly drink staining beverages, do so through a straw and try to cut back. Try to drink more plain water after eating or drinking something staining or acidic to help reduce the negative effects.
Most food does not stain teeth, but if you are a coffee drinker or if you smoke, you can pretty much count on having discolored teeth over time due to thinning enamel and/or staining. If this is the case, then having your teeth cleaned every three months may be in order, besides trying some of the natural remedies listed here.