A "better" quality Flannel Backed Tablecloth, straight pinned into my wall. I say better, because one of my classmates had one that had hardly any flannel on the back & her squares did not stick very well.
This came from a recent quilting class, as the instructor wanted us to bring one, BUT I also have a Fons & Porter book, American Country Scrap Quilts that was published in 1995 that talks about using a tablecloth "way back then".
Happy Quilting Day!
Susan