I’m pretty organized. I think it’s a habit that comes from an art/design background. Any old stuff is still very much around, in its own proper folder. I keep everything by event and date, so I always know which file is my newest draft.
My writing folder is, for the most part, set up by Project Name > Year > Month. For NaNo, I tend to have it under NaNoWrimo > 20XX, with a separate subfolder for Camp NaNo. I usually have subfolders for notes and reference files.
This way I can look at old drafts, notes, and whatever else I’d saved. Sometimes it helps, and sometimes I just cringe. lol
Of course, I’m mostly digital, as evidenced by the above. Handwritten notes I still organize by different notebooks and sometimes binders if the project is big enough. I don’t often throw notes out because I love printed/handwritten notes to glance at when I’m typing the actual story on my laptop. The only time I’ve thrown notes out was when I retyped them, but I usually end up printing them right back out, anyway.