No matter what you are doing, if you're using a medicated or chemical agent, you should always follow the instruction precisely. Have a timer with you when you are whitening your teeth and make sure you do not leave the strips or trays on longer than the instructions say to or you can ruin the enamel coating that protects your teeth naturally. Some people can experience tooth sensitivity where anything that touches their teeth causes extreme pain and awful sensation. Obviously, if you have used teeth whitening methods and this happens, stop immediately and visit your dentist or use Sensodyne toothpaste. You should not be using tooth whitening kits if you have receding gums, sensitive teeth and gums in general, or are allergic to peroxide (active ingredient in tooth whitening kits), but you should discuss it with your dentist before you go ahead and attempt doing so on your own. It is also important to note what kind of teeth you have because brownish-tinted and grayish-tinted teeth do not bleach well and if you have obvious fillings, bleaching will do nothing for their color but make them stand out even more in comparison to the rest of your newly-whitened teeth.
An easy way to make sure your teeth are white is to follow a proper diet heavily infused with fruits, vegetables, and drinking plenty of water. If you drink red wine, pop, caffeinated drinks, or colored drinks, you will diminish the whitening result fairly quickly. It almost goes without writing that if you smoke cigarettes, you should quit for a myriad of reasons, one of them being that they stain your teeth permanently and whitening kits won't do anything for you.
You may have other, associated issues causing your teeth to change color, for example, if you suffer from Bruxism (tooth grinding) you will wear away the bottoms of your teeth and inside the chipped areas you will see yellowness. You should go see your dentist if this is the case for you so that they will provide you with a custom mouth-guard to be worn at nighttime when you cannot control your jaw's motions.