I’ve known about the Command-Shift-3 (and Command-Shift-4 in OS X) shortcut to take screenshots of a Mac for years. That said, I always wanted a way to save the screenshot to the clipboard. The answer was right there in front of my face the whole time!
Command-Shift-Control-3 and Command-Shift-Control-4!
Knowing this would have saved me countless hours in the past!