For my bedside reading material I mostly use magazine inserts, since that’s where I tend to read my SF and mystery magazines.
For the books I carry around, I have several Last Line Bookmarks, which are convenient to keep track of exactly where I was when the bus arrived at either where I was waiting or where I was getting off.
I use the little page-flagger Post-It notes like this. They’re small, so they don’t get in the way of my reading and they stay in place. They’re reusable, but if I leave one in a book when I’m done with it it’s no great loss.
I place the book face down, or use a pen.
Generally, the little business reply cards you find in magazines. If not that, you can pick up all you want at any science fiction convention (I’m not convinced a bookmark promoting a book ever sold a book, but all small press authors make them). I also have a few leather bookmarks given to me as gifts.
For To Do lists and such I like to use vertically ruled 3x5 index cards. Which are hard to find, so I buy packs of regular (horizontal) ruled 5x8" index cards.
After a few minutes with the paper cutter my 100 big cards have morphed into 200 3X5’s PLUS 100 2X5’s.
I have a lifetime supply of 2X5 bookmarks. 
The trick is to laminate the bacon 
I am currently using a football game ticket. It’s the exact shape and size of a traditional bookmark, however much more expensive at $70.
I tear off the top of a matchbook cover or use the plastic phone cards from Tracphone.
As usual the Family Circus has already address this
What is up with you and The Family Circus?
A single square of toilet paper folded in half or else a small scrap of paper. I’ve been thinking lately about getting some business cards just to be able to use them are bookmarks.
If I’m not just grabbing whatever’s around, I use an old baseball or football card from my childhood collection. The way it works is that I use a card with a face shot instead of an action picture, and then wherever the player is looking (i.e., where his eyes are), that’s where I left off.
A dollar bill works well, too: it’s the right size/shape, and it’s cheaper than any purpose-built bookmark.
I use bookmarks I get from bookstores or places like museums. I don’t know how many I have, probably over 100 at this point. Back when my book collection was under control I put bookmarks into the next n books on my list.
At the moment I’m using one from a museum in Berlin (of the Ishtar Gate) and one from Death Valley. I still have bookmarks from a bookstore I used to walk to when I was in high school, over 40 years ago.
Bookmarks are great souvenirs - colorful and relatively cheap.
When I go somewhere, I keep the airline tickets, the museum tickets, flyers from a theatre, things like that. Every book at home has that type of bookmark in it. So when I open an old book from the bookcase, it brings back memories.
For books that I borrow (from a friend, from the library) I’ll use whatever is lying around handy, like the list of library books borrowed, or any other piece of paper lying around.
Ha!
Sorry about that.
I’ve collected bookmarks since I was very young. I love them. I buy new ones all the time. Metal, paper, magnetic… the fancier, the better. And I use them… but when I forget to bring one along with me (generally to work), I like to grab a small post it note, and use it upside down, so the sticky side sticks to the page I’m reading.
Usually my airline boarding pass stub. Then when I’m finished I just leave them in the book. Makes for some fun nostalgia when I re-read it years later or give it to someone else and they tell me.
These days when I’m reading a physical book (as opposed to something one my various Kindle apps) I’m almost always in the bathroom, so I usually use a square of TP as a bookmark. Only other things I can think of that I use occasionally are gum wrappers and 3x5 cards.
I rarely read real books anymore, and usually finish them or have no plans to do so when I do read them.
But my mom does something I haven’t seen any of the rest of you mention: she leaves the book open, laying it on the pages themselves. I think it started accidentally, as she always reads before bed, but now it’s how I know what book she is reading.
I often use $1.00 bills but I seldom read more than a couple of books at a time. Since I live alone, the potential for petty theft isn’t large unless I find myself in need of a one dollar bill.
