I had five 3 by 5 fileboxes filled with index cards of notes, source carefully noted on each. I never achieved an electronic system. Twenty years ago I gave the practice up. I now have three boxes left. The other two gradually diminished as I used the back of each card for grocery lists, telephone messages, to-do reminders, and the like. I'm starting on the third box. Sometimes I even read the notes and am amazed at what I used to know. Now I just read and enjoy, and if it's my own book I don't mind writing in it in pencil.