I use a pad and pencil!
Don't leave the house without one.
Ditto. While I concede completely that a great deal of my "writing" takes place in my head either at work, in the car, or when I lay down for bed at night, I've
always got a pad and pen nearby. Too many times I've had a great idea and thought, "No way I'll forget this!" Guess what? Yep...have no clue what those ideas were.
If it pops into your head, write it down. Yes, stories change constantly, whether in minute or broad, sweeping areas, and there's no chance you'll stick to everything you write down. But what about that one great idea that made your story so epic...then you had to go to work...uh oh, Bob called out now I've got to work over...crap, I've got to take care of laundry so I've got something to wear...I thought that gallon of milk was full, got to go to the store...I need to get something to eat...boy, I need a shower...man, I'm exhausted, gotta get some sleep..........oh, crap...how did it go?
Exaggerated? You bet, but also as real as it gets. I applaud your effort at keeping a mental map (yours sounds to be in much better shape than the one I try to keep), but if you're serious about your writing, it only makes sense to get your thoughts on paper so you can take some of the pressure off. After all, it's easier to thumb through a notebook than that file directory in your brain.