Make non-destructive enhancements to shadows and highlights
Open your image and create a 50% Grey layer via Layer>New>Layer. Select the blend mode and change it to Overlay and tick the check box to fill it with a neutral colour. Hit OK. This layer will now have no effect on your base image other than to protect it.
Select the Brush tool and lower the Opacity to 10-15%. Press D to reset your colours to default, then X to switch white to the Foreground colour. Draw in white on the grey layer to brighten areas such as eyes, forehead or hair highlights.
Now hit X again to switch to black. Paint with 5-10% Opacity over shadow areas to enhance the depth, for example eye make-up, around the hair line or defining facial features. To reverse any of the effects, simply paint with 50% grey back on the layer.