Currently I have a little black moleskin book with my dungeons in it. Actually it has sketches, layouts, ideas, and little discriptives like "The Library of whispers" or "Tale of a memory snatcher". This is how I build my adventures. Many times I will draw my ideas before I ever write them. I find a picture can hold so much more of my ideas than just words. Later I will work out the details, match the maps, figure the DCs and stats, and fill in the story. But even then I always use my little book or prints of art while DMing to jumpstart my own emersion into the game.
I wonder how other DMs do it?