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:
Goes for your home, your neighborhood, or something you've borrowed. It can be applied to the planet, the beach, the trail, the car, the job.
Hell, it even goes for people. Leave them a little happier, a little wiser, a little more prosperous than before.
Second rule, give people the benefit of the doubt and don't attribute an action as the person. Did they cut you off on the road? They're having a bad day and made a mistake. They're speeding? Maybe they are on the way to see a loved one without much longer to live. Don't call someone an asshole just because they made an asshole move. People are so much more than that one interaction with them.