It’s really simple for me…
1. I write everything down in an unlined spiral notebook with perforated detachable pages.
2. I file every page into a labeled green file folder in a file cabinet.
3. I keep all of it.
I’ve been doing this for 30 years. I have “everything” I ever wrote. If fills 3 two drawer file cabinets.
I don’t print and save anything which is already stored digitally. I hardly save much else.
About once a month I pull out a folder a go through it. Obviously, there’s a lot of stuff that appears to be of little use now, but I never fail to find “something” of value. I give away or donate any that is replaceable (which includes all books). But not my own writings. I don’t remember how I handled that issue 12 years ago, but I do know that I can find all my notes on it pretty quickly. This way I don’t have to remember every detail, but I always have my younger self and much of my experience as a resource at my disposal.