I use index cards as bookmarks and for note-taking. Post-it's also work. I also wrote down references to pages, paragraphs and sentences, etc. on those cards,

I use index cards as bookmarks and for note-taking. Post-it's also work. I also wrote down references to pages, paragraphs and sentences, etc. on those cards, e.g. p42p3s5. Nowadays I just make a light x in the margins, with a pencil (so that I can erase it if I want to).