Sage advice from the inside cover of the new Dungeon Masters Guide
Disclaimer: Wizards of the Coast does not officially endorse the following tactics, which are guaranteed to maximize your enjoyment as a Dungeon Master. First, always keep a straight face and say OK no matter how ludicrous or doomed the players’ plan of action is. Second, no matter what happens, pretend that you intended all along for everything to unfold the way it did. Third, if you’re not sure what to do next, feign illness, end the session early, and plot your next move. When all else fails, roll a bunch of dice behind your screen, study them for a moment with a look of deep concern mixed with regret, let loose a heavy sigh, and announce that Tiamat swoops from the sky and attacks.