When my son was old enough to leave the house by himself, I had one simple piece of advice. Every time I told him, "Don't be a douchebag."
When I was hugging him in his dorm room after getting him settled in to start college, he whispered in my ear, "I know Dad, don't be a douchebag." I cried as I left.
He's 24 now, is a mechanical engineer and knocks down 100K+/year. Saw him last week and we went out for sushi. When we were saying our goodbyes in the parking lot, at the end of our pleasantries, in unison we said "And don't be a douchebag."
It's weird what ends up being bonding moments, yet in and of itself is good life advice.