The most useful key in Lightroom is the ALT key (Option Key for Mac users). In the exposure settings, holding the ALT key while moving the sliders will show you when you over or under expose some areas of the photo. But it is especially useful when you sharpen the image. If you hold the ALT key while moving the Masking Slider, you will see your mask on screen. The sharpening settings will be applied to the areas in white while the black area is not affected.
If you use it while moving the Details or Radius sliders, you will see another kind of image to help you with the settings. If you do this trick for the Gain slider, you will see your image in Black and White which will help you see more acutely the sharpness results on your photo.