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:
Both. It's like the saying "Governing a big country is like cooking small fish." (With the explanation that if you keep poking it, it'll disintegrate) also taught me how to cook fish as well as realpolitik.
This is a metaphor for life in general. If you find that all your interactions are negative, check yourself. Are you the problem in your relationships and interactions? How can you fix that? Clean your shoes.