I use OneNote for multiple-session games (one page per session, with dates and attendance), paper for oneshots. They get kinda spotty when I'm super engaged, but they're otherwise serviceable.
I also take phone pictures of the battlemats of every session. It takes seconds and makes for incredible nostalgia material later when you go through it. I cannot recommend it highly enough; the feels I get looking through those maps are quite strong, even just a year or two after the fact. It's a good visual memento.
My only regrets are not marking up those maps after the session ends, and not taking photos of the group at each session for posterity.
One of my previous groups straight recorded session-audio on a laptop. I was a bit uncomfortable with that idea, but it's tempting. Like someday you might go back and listen to that, hear what your game sounded like five, ten years ago. On the one hand, recording, but on the other hand that'd make for some powerful moments when I hit the middle/old-age brackets and have some evenings free to remember games of my youth.