Step 1
Find a picture or use your photo. I found this photo on a stock site and will be using it, you can use it or use your own.
http://sixpop.com/images/file/24660719.jpg

Step 2
Make sure the photo has a bit of yellow in it or it's kind of pointless. Press Q (enters quick mask) and grab your brush tool (B) Using a hard or fuzzy brush, brush all over the teeth. Try not to get the lip or skin areas but doesn't matter. Try your best. As you are brushing over the teeth area it would appear red over the spots your brushed over while in quick mask
http://sixpop.com/images/file/44197775.png

Finish selecting every teeth

Step 3
Press Q (exits quick mask) Now your teeth that you selected are now marching with ants.
http://sixpop.com/images/file/4639252.png

Step 4
Go CTRL + SHIFT + I
Click the yin yang symbol in the layers panel (black and white looking) and click Hue/Saturation.


Just move the saturation bar down into negatives until teeth start looking a decent white. Just move it enough so that it looks natural and doesn't look bad.


Step 5
Results for me
http://sixpop.com/images/file/32729780.png