I got a big roll of styrene sheet (card for you UKers, not foam) from the cut ends bin at a local-ish plastics supplier. Something like 10 square feet for a buck. Score!
However, as it was being stored as a roll, it's got a curve "trained" into it now. Even if I cut off a little piece, that little piece has a curve that makes it hard to build clean looking stuff with it. I tried rolling a piece of it against a desk edge like you would with paper, but the curve reasserted itself later after I'd started building, causing the thing I was building to go all warpy.
Is there a way to flatten out the sheet without resorting to putting it in the oven?