Stumpy
Stumpy is the 3-legged dog that started as background flavor in our campaign that I made up on the fly. He has now become a legend in our campaign, even so much that when invaded, I handed out NPC cards to players to control, and people wanted to play Stumpy. In 20+ years of DMing, I've made up innumerable NPCs, but I've never had players latch onto anything like they have Stumpy.
"Stumpy" is a metaphor for remembering to let the game go wherever it may go, to create colorful, meaningful scenes that tag the senses, that NPCs make the gaming world come alive, and to pay attention to how the players react to your world more so than how you originally scripted it.
Since that metaphor won't fit on a post-it note, it has to be Stumpy. My advice: find your Stumpy reminder.