*Press R to get to the Crop Tool. It works even if you’re in the Library module.
*Once you’re in Crop mode press the letter O (for Overlay) to change the overlays that you see on your photo. They’ll toggle between the rule of thirds, a grid, and several other overlays that you’ll never use
*Press A to toggle between constraining/not constraining to the aspect ratio
*Press Cmd-Shift-R (PC: Ctrl-Shift-R) to reset your crop all together.
*If you’ve changed the Aspect Ratio on a previous photo then press the letter S to set that same aspect ratio for the next photo.
*And finally, a really cool way to crop is to go into Crop mode. Then press the letter L twice to go into Lights Out mode. Now you have your photo in crop mode with none of the clutter of the interface. Press L again to get out of Lights Out mode.
Happy cropping and have a great weekend.