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.