Most often, I use the Screenfly Chrome extension for screenshots if I need to save a screenshot of the page locally, or the Joxy extension if I need to send a screenshot to someone. But sometimes I have to take screenshots using the Macbook itself, and I persistently forget the combination of numbers every time.
I will save it, and you will remember (you probably have a better memory than mine):
Shift+Command 4
Four! Four, Carl! Why 4? How does this relate to screenshots? Who can even remember that 4 is a screenshot?