I like bookmarks. Memorizing page numbers gets tiresome and flipping around looking for one's spot in a book often leads to the unpleasant discovery of spoilers, so I nearly always use some form of marker to hold my spot. In a pinch I'll use a Kleenex. In even rarer instances I'll use a book's dustjacket, but that often leads to the disfigurement of said jacket, especially if the book is of significant size. Most of the time, however, I use a proper-ish bookmark.
I have a number of these lying about in my room, some more frequently used than others, some that have never been used at all. Right now I have a 2010 Alibris bookmark in Kidnapped - the colors coordinated and the quote on the bookmark, "There is no frigate like a book to take us lands away," seemed fitting for a Stevenson novel. It has puncture marks in it where Buster has gnawed it. In The Imitation of Christ I have a bookmark with Philippians 4:8. It's cracked in places, also where Buster gnawed it. I keep a pink Beatitudes bookmark with tassels in Faith's Checkbook, and amazingly, Buster has not gnawed that one.
Jenny pointed out in her post that clothing tags double as excellent bookmarks. I only have one of these: an elegant "Lapis" tag with a sleeping fairy on one side and a quote from A Midsummer Night's Dream on the back: "Love looks not with the eyes, but with the mind, and therefore is winged Cupid painted blind." Rather an odd quote to use for a clothing brand, but I like it all the same. The bookmark is fairly new, but I expect I'll use it frequently.
Another new bookmark is one my cousin gave me. Titled "Old World," it has a picture of an antique map on it, a red beaded tassel at the top, and a quote (apparently by Confucius) at the bottom: "Wherever you go, go with all your heart." A little ironic, considering that I never go anywhere, but perhaps it's referring to the world of books.
A heap of others make the rounds through my books: a blue gilded one that says "Friends" (a gift, as you have no doubt guessed, from a friend!); a bland red-and-white "Trinity City Books" one that is only for mystery novels (don't ask me why); yellow and blue paint cards, the one with a canary on it, the other with a hydrangea. A bookmark made of wood or bamboo, handpainted somewhere exotic, is still in Red Moon and Black Mountain, which I have yet to finish. A little blue one, made by a friend and with the quote from The Song of Solomon "many waters cannot quench love," peeks out of Desiring God. If I poked, I would probably find a host of others still hiding in books. I try not to leave works unfinished, but sometimes even the best of intentions fail.
So you see, I have quite the host to choose from when I start a new book. And yet the other day when I had two books downstairs and no bookmarks at hand, what did I do? Tore an index card in half and used the pieces.