Recently, my entire family was reading the same book, all 5 of us. That meant we were all at different places in the book. I don't know how many times someone's page was lost before we thought to use post-it notes as bookmarks.
The post-it notes are thin and don't make the book bulge, even with five of them in there.
You can use different colors, or write names on the post-its. It is also easy to move a post-it from one page to the other while you're reading. This keeps you from removing the bookmark to read around it, and forgetting to put it back in. Also, the other bookmarks don't fall out while you're reading.
This post is a part of Works for Me Wednesdays. (I LOVE her idea for decorating with kid's art!)