I did not realise how funny I am about bookmarks until this thread made me think about it!
I habitually collect bookmarks of all kinds (I often buy them as a small consolation after it has taken me several hours browsing to remember that I really, really don't have the money/time/shelf space for yet another book!) However, these never seem to make it into the books that I am actually reading at the time, but instead adorn previous reads. I use them to mark favourite passages and things (so that eventually they naturally fall open at those places, oops), and they lend extra decorative effect to the display (or shrine) that is My Bookshelf. I also tend to match them up, so for example an old-fashioned fabric-bound classic might have a similarly antique embroidered sampler.
With books that I'm actually in the process of reading, I was going to repeat what lots of people seem to say, that I use 'whatever is to hand.' However, I've realised this isn't true - I use gift tags from presents, a nice leaf I found, a completed crossword torn from the newspaper, train tickets from memorable excursions but not from my everyday ride to work... Meaningful stuff I guess.
It actually slightly appalled me (irrationally, I know) to learn that a friend of mine (who loves and reads books possibly more than any other person on the planet) took no care whatsoever over what she used to mark her place: price tags from recently purchased socks, the label ripped from the outside of a pop bottle, etc. Having said that, I did once keep a chocolate bar wrapper, as it had a winning code in it for the galaxy book club promotion - although I did wash it to prevent grease stains when I then pressed it inside - that's right - Charlie and the Chocolate Factory So there is some kind of method in my madness, logic in the loony-bin.
PS. Apparently I like not only reading, but *rambling too!