If you have a photo where the teeth are looking a little dull, whitening the teeth in photoshop can really improve the photo. This is actually fairly quick and easy to do.

First, zoom in so that the teeth are filling up the better part of the sceen.

Second, select the Dodge tool. Set the Range to Midtones, set the Exposure to 50%.

Third, run the Dodge tool over the teeth. This should only take two or three passes.

To tell if you are done, zoom out to regular size. If you need more whitening, repeat the process. If you need less whitening, undo the previous changes and try again. But this time reduce the Exposure level down (maybe 30% or so). Once you get the hang of it, you can improve a lot of pictures in this manner.

You can use this same effect to whiten the whites of the eyes too.