One of my habits when reading a book is to not simply underline or highlight lines or sections I think particularly good or important, but to jot a little note down in the back of what the quote is about and what page it's on. I can usually remember who said something I'm trying to remember, and frequently I can recall what book it's from, but having the notes in the back is easier than flipping through the whole thing.
Here's a line I scribbled down in the back of "The Prophets", by Abraham Joshua Heschel:
"There are no proofs for the existence of the God of Abraham. There are only witnesses."
(Rabbi Heschel marched with MLK in Selma, and is one of the most important Jewish theologians of the 20th century. I highly recommend reading him. "The Prophets" is quite thick, but his book on the Sabbath is a quick read.)
Do you have any note-taking strategies when you read a book? I tend to do this more with non-fiction, but sometimes a novel will have a line or section I have to notate in the back. What do you do? Highlighters? Endless dog-eared pages?