A quick and affordable way to whiten teeth is to brush them with baking soda. Baking soda is very cheap and is likely to be sitting in your kitchen at this very moment. Now the taste is unpleasant, however you'll see great results if you stick it out. Julia Roberts brushes her teeth with the stuff daily...so that's got to tell you something, right?!
Now all you need to do is dip a wet toothbrush into some baking soda and start brushing. Be sure to only brush for two minutes max, as the baking soda can begin to erode tooth enamel. For these reasons it is also important to avoid doing this treatment regularly - stick to a few times a year, for a week at a time.
To keep teeth pearly white on a daily basis, try using a whitening toothpaste that contains a little baking soda, like the Arm & Hammer Advance White (£2.99 Boots / $2.99 drugstore.com).