"Be nice to people on the way up; because you just might meet them on the way down." What's the best piece of advice you've ever received?

"Write code as if the one who takes over is a psychopath who knows your address."

First programming teacher I ever had gave me that advice, except instead of psychopath it was an axe murderer. I remember it often but rarely follow it (writing cleverly obtuse code is fun)!

This reduced me to uncontrollable giggles!