The best results come from laser whitening which is performed by a cosmetic dentist. It can make your teeth 4 to 6 shades lighter within an hour after visiting your dentist! It is expensive though and typically runs over $1000! The only potential side effect is sensitivity.
The next most effective way to whiten your teeth is by using trays and gels. Your dentist can make a set of trays that fit your teeth with a mold. You then add a gel with a bleaching agent to the trays and leave them in for up to an hour. You will need to repeat this for a few weeks before you see the results. I do this every once in a while and sometimes my gums swell up!
Whitening strips can be found in drug stores everywhere now. They take a bit longer to produce results but they are so easy to find and are very inexpensive compared to the other treatments.
The Zoom Whitening Pen is a click pen than can be taken in your purse or pocket and applied after you eat or drink something that stains your teeth. I dries quickly but comes at a price. A set of two is $45 but each pen lasts for about 30 applications.
Of course, after you whiten your teeth you need to do your best to keep them that way! The obvious staining culprits are tobacco, coffee, tea, and colas but did you know that grape juice, red wine, blueberries, soy sauce, and curry can stain your teeth too! Basically, if it can stain your clothing or carpet, it can stain your teeth! The best way to avoid stains from these things is to drink them through a straw which reduces your teeth's exposure to them and brush your teeth right after you have them.