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.
There are other games than DND. Play more of them and steal ideas from them.