"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."
This reduced me to uncontrollable giggles!
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)!
"Think before you speak." A platitude that is well-worth understanding.
This is an excellent one for colleagues (!)
"Know your future self" Not necessarily the best advice ever but one that becomes more and more relevant with age.