Because we usually have books strewn about the house, we also usually have bookmarks strewn about. I use the term "bookmark" loosely. Anything that marks the place will do, as long as the book isn't left open, face down, squishing the poor binding. (Anathema.)
A receipt. An envelope. A gum wrapper. They all do nicely. Bethie's favorite way to mark her place is by using a postcard. I have a two-inch stack of old postcards ("old" as in from the 60s) that she has confiscated and uses liberally. I like to think she's also learning a bit about geography as she flips through various antiquated shots of San Francisco, Mt. Rainier and the Redwood Forest while reading Nancy Drew.
One of my favorite ways to mark a book is by using apparel tags. Some of them are so very pretty. Have you noticed? It seems a shame to toss them. I feel so very frugal when I tuck them between the pages of my latest read. And they're nice and durable, too.
I've now gotten my kids on board, and they're quick to set aside promising bookmarks. Especially Aidan.Which can lead to interesting results.