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:
My wife, who gave two births, was in a car accident and fought through hell to survive, still rate kidney stones as the most painful thing in her life.
I said would she rather take a bullet to the leg? She said she'd take two over a kidney stone.
All of this can be resolved just by drinking water.