If you need to mark your place in an actual, physical book, any old piece of paper will do: a business card, a boarding pass, a napkin. However, for a more efficient solution, reader Simon suggests using Post-It Flags to keep track of where you are.
Here's Simon's explanation in his own words:
I've always had the issue of not having a bookmark and having to fold the page over, and upon finally getting a bookmark it often gets lost very quickly, and I'm back to damaging my books. Thanks to these small post-it stick notes/flags, I have no longer had any problems; they are simply the best bookmarks. Firstly, they're cheap which is a bonus, making it no issue if they get lost. Secondly, they're sticky, so they don't get lost so easily. Thirdly, they're light; and fourthly, you can stick it to the exact part where you stopped reading so that you can carry on reading from where you left off. It's really useful and a great 'life hack'!