from Ryan Holiday (likely via Austin Kleon):
-I’ve talked about this before, but the key to this system is the ritual: Read a book or an article and diligently mark the passages and portions that stand out at you. If you have a thought, write it down on the page (this is called marginalia). Fold the bottom corner of the page where you’ve made a note or marked something (alternatively, use post-it flags).
-A few weeks after finishing the book, return to it and transfer those notes/thoughts on to the appropriate note cards. Why wait? Waiting helps you separate the wheat from the chaff. I promise that many of the pages you marked will not seem to important or noteworthy when you return to them. This is a good thing–it’s a form of editing.
-In the top right hand corner of each card, put a theme or category that this card belongs to. If a card can fit in multiple categories, just make a duplicate card. Robert uses color coded cards for an extra layer of organization.
— The Notecard System: The Key For Remembering, Organizing And Using Everything You Read
Holiday also has interesting notes in his suggestion to keep a commonplace book.