Photoshop Tutorials: Whiten and Brighten Teeth In Photoshop
Learn Adobe Photoshop with Photoshop Tutorials at Photoshop Essentials.com
One down, one to go:

Step 8: Brush With White Over The Woman's Teeth
Now that the guy's teeth are done, I'm going to use the same Hue/Saturation adjustment layer to whiten and brighten the woman's teeth. First, I'll zoom in on them so I can get a better view of what I'm doing by holding down the Ctrl key (Win) / Command key (Mac) and the Spacebar and then dragging out a selection around her mouth:

All I have to do now is the same thing I did with the guy's teeth. With my brush tool still selected, white still as my foreground color, and the Hue/Saturation layer mask still selected in the Layers palette, I'm just going to paint over her teeth to whiten and brighten them. As I paint, I'm revealing the effect of the Hue/Saturation adjustment layer on that area:

Again, I can press X to switch my foreground color to black if I make a mistake and need to paint back over an area, then press X again to switch back to white. And to change the size of my brush as I'm painting, I'm using the left bracket and right bracket keys. Finally, for darker areas in the corners that don't require as much brightening, I can lower the opacity of my brush up in the Options Bar.
I'll just continue painting over her teeth until I have them all looking whiter and brighter:

And there we go, they both now have whiter and brighter teeth:

One one thing left to do...