Let's see. When I used to read magazines, the subscription cards were very handy as dwndrgn has already mentioned. And I think it was Brian that covered receipts. That's about it for me.
I won't dog ear books either. However, it's usually very evident that I've read them if I own them. Not novels, so much, although I will mark passages that I think are particularly well-written; but when I'm reading non-fiction I always read with a pencil in my hand and pretty much constantly mark passages and make comments in the margins. Part of this is a holdover from being a student, but most of it is the fact that this is the way I interact with books. To me, reading - again, especially non-fiction - is an active pursuit as opposed to a passive one, and I talk to - and back to - the writers of the books I read all the time by carrying on the conversation in the margins.