You can probably fix it with the stunning, amazing and truly awe-inspiring paperweight GW puts out called "warhammer fantasy: the rulebook". Weighing in at 5ish pounds, this book will become useless in another couple months as we ramp up for 9th edition fantasy.
Ok, in all seriousness, a heavy book keeping the mat flat will probably help, but I'm sure you won't want to do that before every game. Try flattening it out, then roll it up loosely with a stretchy rubber band.
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