1. Once you have your photo opened. Duplicate the layer you are working on by pressing CTRL + J on the keyboard (command + J on a Mac). This way if something terrible occurs, you simply have to delete your duplicated layer and not have to worry, not that this would happen of course! .
2. Zoom into the mouth, preferably at 100%. You can do this by pressing CTRL + ALT + 0 (or command + option + 0 on a Mac).
3. Take the lasso tool (3rd tool down on the default tools bar) and simply select (draw) around the teeth trying to be as accurate as you can. Another way to quickly access the lasso tool is by pressing L on the keyboard. You do not need to select each individual tooth, just the teeth area. When you are finished, connect the start and end points to make your selection. You know you have made a selection when dotted lines (marching ants) circle your selection that you have made.
4. Go up to the top and select Image > Adjustments > Hue & Saturation
5. On the second pull down menu (where it says Master), select the Yellows channel.
6. Slide the Saturation slider all the way to the left. This will turn off the color yellow within your selection and bring out a whiter smile.
7. If you are satisfied at this point with your teeth, then you are finished. If you would like an even greater shine and brighter whites you will need to use the dodge tool.
8. Press O on the keyboard (or select the 14th tool down on the tools list).
9. Up at the top, for the options for the tool, choose an appropriate brush size (something smaller than a single tooth) and set the range to midtones and the exposure to 7%. Make sure the box labeled Protect Tones is checked. A higher exposure will have a brighten teeth quicker, but the idea is to make subtle touchups.
10. Next paint over the teeth in back and forth motions (brushing). You may not notice it, but the teeth are getting whiter. To see the before and after differences, go back a few steps in your history to before using the dodge tool. Shinier huh?
11. Again, do not over whiten the teeth. Keep it (and all of your other retouching) subtle and natural looking.