Montaigne kept what he called a “common place book” — a book of quotes, sentences, metaphors and miscellany that he could use at a moment’s notice. I keep a more modern — but still analog — version of this book. I write everything down on 4×6 note cards, which I file in boxes. (You could do this digitally, I suppose, but the physical arrangement — being able to lay them out on a desk — is critical to my work). I picked up this system from Robert Greene, who also used it for his books.