Take it to a framing shop (Ben Franklin and Hobby Lobby stores usually have one) and ask them to dry-mount it for you. They will mount it on a piece of foamboard, which will flatten it out about as much as you’re going to get, and probably still reasonalbly presentable. Mr. S has a map of Middle Earth that was folded in four pieces; we had it dry-mounted and while you can still see the fold marks, it looks OK.