Flattening Cardboard

[Mr. White]

I picked up some pieces of road for GW's Dark Future, but they've got some warp or bend to them.

Any suggestions on how to press or flatten these things so they lay flat?

The following is a pic to give you an idea of what I'm talking about. NOTE: these are not the pieces in questions. This photo is from the Realm of Chaos 80s blog.

[Elk101]

What about ironing it using the steam setting (on the rear and using a tea towel or something over it) and then flattening it with something heavy for a few days!

[Ahistorian]

Large piles of heavy books for a few days would also be good - that's what I use. Maybe iron them and then leave them under a weight until they've dried?