For me, it was: “If it’s going to help your players have more fun, cheat. Fudge a die roll. Make shit up. The dice don’t tell you what needs to happen, your players’ reactions do.”
Obviously, many people will disagree with this, but I’ve always appreciated this advice, and I believe it has made me a better GM.
Get your players to tell the story.
As a GM, I’m not the only story teller at the table. I ask my players to provide backstory, flesh out NPCs, explain how/why their characters can do things that aren’t on the sheet.
It takes a lot of load off me, and it makes it more fun for everyone.