I tried plasticard, but I use a lot of non-plastic materials in my terrain and they wouldn't stick and hold.
So I use masonite, very cheap and available in large sheets from any hardware store. And regular construction glue will hold anything in place on it nicely. You just have to watch make the base piece too large, as the masonite can warp and is a bugger to get flat again without breaking some of the glue seals. There's some pics in the Distractions section, if you are interested.
On another note, Battle in a Box covers the base of their area terrain pieces, hills, etc., with felt. I find that a very nice touch that really helps keep pieces in place.
Good luck, and yes, post some pics.