I'm using a hollow door, the kind with the cardboard honeycomb inside. I check it every once in a while with a long straightedge just to be sure, but it has stayed good and flat for me. (Check it lengthwise, widthwise, and diagonally in multiple places each.) Mine was a throw-out from a neighbor's remodeling project, so it previously saw years of use as a door (of all things) and has a big hole in it for wires to pass through (once used for a doorknob, I suppose).

For surface, I used "sheetrock" for a time, but got tired of the force it took to insert a pin. Now I use ceiling tile, which is much easier to pin to but still holds the pins quite well. Homasote would have been better yet -- also easier to pin to than wall board but doesn't seem to lose its pin holding capability over the years. Great stuff.

Something else I've found invaluable is gravity conversion holders. I have a set of machined square, gravity-actuated steel workpiece holding devices: several are 2"x2"x4", a bunch are 2"x1/2"x4", and a few are 12" - 18"x1/2"x4". These things convert a little gravity into a lot of holding force, especially the larger ones. I really don't know why more people don't get a set of them, but then I never had any idea how good they are til I got mine!