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:
"What to do if you find yourself stuck with no hope of rescue: Consider yourself lucky that life has been good to you so far. Alternatively, if life hasn't been good to you so far - which, given your present circumstances, seems more likely - consider yourself lucky that it won't be troubling you much longer." Douglas Adams