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.
Similarly “The DM rolls the dice because it sounds good. Occasionally the dice tell an amazing story, unfortunately the DM must always tell an amazing story.”
Unfortunately, the first lesson I learned from GMing is that the story I planned is shit. The only reason these stories are worth telling is because the players make decisions you don’t expect and the dice send things in crazy directions. If there is no chance of failure, success is meaningless.
Instead, I let the dice tell whatever story they like, and I turn that into a better story than I could come up with. And if a roll would genuinely make things terrible if they didn’t roll a specific way, I don’t roll at all.
I tried fudging once. The players could tell, and the story lost all tension. Now I roll in the open, and every roll is exciting.