Bright Smile Technique - First outline the teeth with the pen tool.
Then make a selection. Then feather that selection a few pixels according to resolution to have a more natural/smooth transition.
Then create a curves layer. With that active, use the whitepoint eyedropper and select a part of the teeth that should be white. Lets just say here the middle of the front of the teeth.
Then turn the opacity of the curves layer down until it looks more natural.
Bright Smile part 2 - Now the teeth look unnaturally white. So turn down the curves layer a bit.
Bright Smile part 3 - Now the teeth look more natural and call less attention to themselves.
Time to fix the eyes, blotches on the skin, etc...
retouching - This shows how I have removed the background, put a new head on the model's body with a better smile and changed the color of the shirt to a more Kellogg's red.
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Photoshop Retouching