One of the most common retouching tasks you'll be faced with is whitening teeth. Like many things in Photoshop, the key is applying the correct technique in moderation to obtain a realistic result. This helpful tutorial will show you three such ways to do just that.
Coming to you from PiXimperfect, this video will show the benefits and drawbacks of three methods of whitening teeth in Photoshop. As Dinda mentions, there's a difference between whitening and brightening. Probably the most common mistake I see with retouching teeth is simply brightening them (or over-brightening in tandem with whitening), which makes for very artificial results that really pop in the wrong way. The trick is focusing more on reducing the amount of yellow in the teeth while maintaining roughly the same luminance levels. I personally prefer the last method Dinda demonstrates, as it allows you to not only zero in on exactly the range of hues you want to adjust (and tweak them later if you so desire), but it obviates the need to brush them in super precisely. Any of the three methods should work well, though; just pick whichever you prefer and be sure to use it in moderation!