Roll it up and rubber-band it for a day or two. Be sure that the folds are 'rounded' and not just folded when you roll it (if that makes sense, easier to show than explain). Then lay it flat and put some books on it. In a day or so it'll be ok. As per the above suggestion, if you need it quickly, use a hair-dryer to warm it up and flatten it out.
I have a few mats that came folded or tightly rolled and the books worked just fine. I also store them flat, I use binder clips and clip 3 or 4 (depending on the weight) mats together and then hang them on the wall.
When I need to transport them to a game outside of my home base, I roll them into a 3" dia x 36" cardboard mailing tube, but roll the side you want to use on the outside. Then you can lay it flat and blue-
tac the corners if need be for a bit. As play goes on, it will stay.
Hope that helps!!