To help speed up your workflow, Adobe has conveniently created shortcut keys so you don’t need to look through menu's to perform actions. Below are some of the shortcut keys I have found useful and hopefully you will too.
Undo - Ctrl + Z
Inevitably you will set something you either don’t like or didn’t meant to and the quickest way to undo that is by pressing Ctrl + Z. This will undo the very last action performed.
Show/Hide Clipping - J
This is a quick way of working out if you have clipped either your highlights or you shadows. Simply press J and this will turn clipping on. This will show red where the highlights have clipped or blue where the shadows have clipped. To turn this off press J again.
View Before and After - Y
By pressing Y you can instantly view the original image (before) and the image you have been working on (after). This allows you to compare the two images side by side. Press Y again to go back to editing.
Cycle Crop Overlays - O
Pressing O will cycle through the different crop overlays available to you. The default will cycle through the following
If you only want to cycle through a couple of these you can go and customize by going to Tools > Crop Guide Overlay > Choose Overlays to Cycle.
Tip: By pressing Shift + O you can cycle in reverse order
Lights Out Mode - L
Lights out mode will dim the Lightroom workspace so that you have no distractions when viewing your image. Press L will cycle through 3 modes.
Note: The dim level is customizable. To change this go to Preferences > Interface > Lights Out > Dim Level.