If you had to give one piece of advice that is pretty much universally applicable, what would it be?
I wouldn't dare defile Douglas Adam's memory by not mentioning that you should keep a towel with you at all times, but my second contender is a surprisingly short three-parter:
Think before you speak, I speak without thinking and it's got me in trouble for petty things. Using WE over YOU is extremely valuable.. even when you had nothing to do with the problem it is OUR problem.