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:
Develop a treatment scheme that works for them as it would for you. The first step in treating others the way you’d like to be treated, is finding out what they like.