I use post-it notes for many things and one use I especially like is using it for is annotating library books. I often find sentences or pieces of sentences or even an entire paragraph I want to remember, but I don't want to jump up each time and type it into a Word document. Instead I place a small stack of Post-It notes on the inside front cover of the book and when I come across a great quote I just put one right where the quote begins. It doesn't slow up my reading. If I have a pencil handy I might add a few words about why I like it. When I'm done reading the book it's easy enough to transfer the quotes.
I do this for fiction as well as non-fiction, for books of projects and poetry books too.