Dried corn cobs, large leaves, small leaves (more of them), dried grass, sand, rocks (thanks to the Ojibway indians for this one), telephone book pages, old newspapers and magazines, the Sears catalog, and lastly...the ass rag.
All of the above assumes that you have some kind of an outhouse or an army style burn barrel set up. If you want to get away really cheap, there are plastic toilet seats that are made to fit on the top of a 5 gallon bucket. Use a two garbage bags inside the bucket.