Firstly, start by finding a photo that you can demonstrate the effect to. Try to use Google Images to find a good picture or feel free to use mine. Now open up your photo in Photoshop, you can either keep it at it's current size of resize it down a little bit. Then select the Polygonal Lasso Tool to create the selection as on my picture below.
Duplicate selected area with Ctrl+J. We will experiment with this copied layer. Apply Image > Adjustments > Levels to change hue and saturation for current layer. Use settings similar to these:
See the difference now:
Ok, looks better, then before, but it will looks better without pink lips outside the teeth, get out the Eraser Tool and a sharp round brush to clear area out of teeth.
To finish off the tutorial, I think, we need to remove purple hue on teeth. Do this by using Image > Adjustments > Hue/Saturation with next parameters:
As for me, looks better now!
Ok, after that select the Dodge Tool (Brush: 45px, Range: Midtones, Exposure: 50%) and make a little dodge work on the teeth in the mouth corner.
Well, we're done for the tutorial! I hope you enjoyed it, even just a little bit. Now, you have real 'Hollywood smile!'
More Tutorials: 
