The best DO advice:
"Be fair and neutral in setting up situations and then let the dice fall where they may."
"If they want to spend time playing through minutae or chasing red herrings, let them. There's no rush."
"If they want to fight each other, let them - as long as it stays in-character."
"Never interrupt an in-character conversation unless the game state forces you to."
"Learn how to hit the curveballs they'll throw."
"Adhere to precedent."
"Make the setting believable, lasting, and consistent with itself as a real place the characters inhabit."
The best DON'T advice:
"Don't show favouritism to any player or character."
"Don't complain when the players throw you a curveball, as their doing so shows engagement."
"Don't make a ruling you're not prepared to live with for the duration of the campaign."
"Don't lock in your story or prep, keep it malleable to account for what happens in play."