I carry a 10-pack of pencils with me. It triggers some snickers at work, but I always felt they helped me retain more than the notepad app on my phone.
I love how they let me keep clues to my thought process. I can see the spelling mistakes, the scratched out lines, the thinned spots on the paper where I erased the contents a half-dozen times to get the right word. They are hints that bring me to the string of thought that lets me truly understand what I was thinking when I wrote whatever it is I wrote.
Pencils are like a version control system where someone kindly took the trouble to check in their half-finished code. A repo with the half-baked ideas still there, letting me see the journey from start to finish.