Flannel - its easy to use, cheap, and you can take it down and wash it whenever you want and not worry about what condition it's in after. I agree with QuiltE - make it as large as possible. Mine is about 7 1/2 ft tall and 8 ft long and I LOVE it. I used an off white flannel as I didn't want it to affect what I put on the wall, and its light color is good for my room....plus being in the Pacific Northwest (rainy, long winters), I don't need something dark there.