What's your one DM "trick" that you always tell other DM's about?

Running the Game
Ways to use Solo Monsters
Two Page notes in Practice: For when lazy GM is too little prep...
My 3-year campaign ends tonight. Here's some things that worked well.
r/DMToolkit - How to Easily Describe Scenes: A Breakdown (Blog)
Descriptions for a Clearer Picture 2
Descriptions for a Clearer Picture
Give your players the Tools to be Cool
Gambits, a simple system to spice up combat
Manage Combat Like a Dolphin (imagery to mechanics to imagery)