The Dodge Tool is great for whitening and brightening teeth and eyes!
1.) Open an image of your choice and duplicate the layer.
2.) Select the Dodge tool and set the Range to Midtones and Exposure to 50%
3.) On the duplicate layer, zoom in and brush the teeth and whites of the eyes with the Dodge Tool occasionally zooming out to see the effect.
Be careful not to overdo it, the Dodge tool is very powerful and should only take a few strokes.