I've had my silpats a couple years now, and always stored them rolled. (Don't really have room anywhere to store them flat.) I just secure with a rubber band that isn't real tight. Anyway, I haven't had any problems with creasing or other marks, and they flatten out nicely once unrolled. Hope that helps!
I store ours flat, right on top of the clean cookie sheet pans. Rolling them is fine too, I thought...but their website actually says not to.
The fiberglass Silpat (orange trim for baked goods) will last for a long, long, long time if you treat them carefully and never cut on/bend them.
http://www.silpat.com/index.html
The cheaper versions out there are sometimes okay, but tend to curl up at the edges and wear out faster imho. 
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