The ideal textbook bookmark is the one I lost (I know it's around somewhere, probably in an old textbook). Two inches wide, eleven inches long, and lightly plasticized, it came in an ad for The Economist. The paper was the same weight as the stock used in the subscription postcards that come in magazines. Subscription cards are okay bookmarks, but they're too wide. If I weren't so lazy, I could cut them in half and they'd do pretty well.
The important thing is the weight of the paper. It should be thicker than the paper on which your book is printed, but thin and flexible enough to bend with the pages, or else it just falls out. For textbooks, you want a longer bookmark. A short bookmark tends to drop down so that its end is below the edges of the pages, thereby making it impossible to find your place and defeating the whole purpose of using the bookmark.
A word on fancy bookmarks: NO. A rectangle of plastic or cardboard with tassel is too thick and the tassels get dingy. The heavy paperclip kind are too thick and rip the pages. Besides, my cats will chew on tassels and doodads and pull the bookmark from the book.