About 20 years ago Walden Books sold a line of bookmarks that were colorful postage stamps three in a row in very heavy laminate. I purchased about a dozen because they were just perfect for my needs and I still use most of them.
Ctrl-D
What? Oh. If it’s my own book, I’ll make a slight fold in the relevant corner (smallish fold points me to the top of that page, slightly larger fold to the top-middle; there are four corners to direct me to eight general areas of a spread). (ETA: I got that pedantic over the mark while reading The Naked and the Dead. Lots of books have spoilers/plot twists that are easily picked up in skimming to find your place. That one was particularly poignant)
Yes, I fold the pages of my books. Get over it. It’s not like it’s a library book. For those I stick a piece of chewed gum between the last pages I read so it easily opens to where I was.
Me too! I’m glad I’m not the only one who does this.
Low-denomination foreign currency notes from places whose money is worth far less than yours are also good bookmarks IME; as you say they bring back memories of the trip and they’re usually distinctive so you don’t misplace them as easily. 
Because I read a lot of library books, and because I usually re-sell the books I buy. (Does anyone else un-dogear pages when you encounter them in a library book?)
I actually make bookmarks – I have a little laminating machine, and periodically haul it out and make a batch, with pictures I cut out of magazines, or stickers, or bits of ribbon or pretty paper, or patterns I stamp, or etc. I’ve also got a few souvenir things that I’ve laminated – ticket stubs from museums I’ve visited on my travels and the like.
I also have another half dozen or so giveaways of various sorts (promos for books, bookstores, my local library, art shows, etc.)
So at any time I’ve got a half dozen books around the house with nice bookmarks in them, and a stash of another dozen waiting to be pressed into service.
I think Bill Keane’s estate has forced Marxxx to not explain the specifics here.
I use playing cards. Any deck with a lost or damaged card becomes a deck of bookmarks. Even if you don’t have any partial decks around, spending 2-3 bucks for a box of 52+ durable bookmarks isn’t bad, especially if you tend to lose them a lot.
bookmarks (if I can find one)
due date slip for library books
rubber band/hair band
paperclip
index or similar card
yarn
Whenever I give someone a book as a gift, I buy a bookmark to go with it.
Train tickets mostly. Also receipts.
(Oh yeah, business cards too)
Scrap paper
It used to be a notepad sized piece of paper on which my daughter had drawn a dog’s paw print. Now it’s a commemorative bookmark made by girl scouts for a readathon in honor of a friend’s daughter who died. I got it laminated.
When we’re on vacation we buy bookmarks from wherever we are. Always cost less than a dollar each and they are great gifts to our friends who all love to read. We of course buy them for us too. 
Post-it notes. Or scraps.
I buy Magic cards about once a year and a pack will last me about a year. Occasionally I’ll find my old bookmarks when I reread and it will expand my stack for future use. I also use the receipt for the book if I start reading before i get back around my stack of cards or if i start reading on the john then I’ll use TP. Unless I lose it I’ll use whatever I started the book with the whole time.
30 years ago or so, my mom’s office had a big pile of bookmarks that on one side had contact info for some relevant government or academic scientific department, and on the other said BOOKMARK in very big letters. They were printed on a lovely distinctive burnt-orange-brown cardstock with a laid texture. I picked up fifty or so of them after they’d languished in the office for several months, and still have one or two left. Several years later, after most of them disappeared, I printed some of my own – also on nice cardstock, but I went a little crazy and put *BOOKMARK *on *both *sides. Ten-up on five sheets gave me 50, and I still have five or six around. It’s prolly about time I made another batch.
I use a ticket stub from when my favorite team, the L.A. Lakers, came into town to defeat the local sports team here a couple years ago. Once that one is torn up and unusable, I have the stub from last year’s Lakers victory also.
And here, I thought I’d be the only one to say Magic cards. Thought I don’t buy them for that purpose. I have about a cubic foot of basic land cards and I just use those. (For those that don’t know the game, a basic land card comes in every booster pack, and several come in the premade decks. You need some of these cards to play, but eventually you reach critical mass and get so many that they are effectively garbage.)
Mostly, tissues and toilet paper squares (unused - I may be trashy but I’m not feral).
I have other real bookmarks that I use but I tend to return the books to my bookshelf forgetting they’re in there, and only find them again when I re-read. That’s how I’ve come to be currently using a cross-stitched bookmark that I made probably 14 years or more ago. Since I rediscovered it I’ve remembered to move it from book to book for the last few weeks… but the day is coming when it will go back to the bookshelf to sit undisturbed for several more years. Hopefully I’ll find one of the other cross stitched bookmarks that are out there somewhere - I know I made at least two others.
I don’t use bookmarks. Never have. I just remember the page I’m on.
ETA: Yes, I do this despite reading anywhere from 1-4 books at a time. No, it’s never been a problem.
Had to share: I found my bookmark cross stitch pattern book on eBay! And looking at the cover I realise I must have made more than three. Great… there goes my afternoon, flicking through the pages of my old books looking for forgotten bookmarks 
I like the art on the creatures so I’ll start off with the most powerful creature in the pack and go down from the the lands are the last things I use, right now I’m using some 4/4 black critter.