I have a huge one that is stored behind a chest of drawers. But my 12 x 18 and my 24 x 30ish hang in my closet. I use those plastic hangers that have two clips on them - the kind shorts would hang on at Walmart. I haven't had any warping problems at all.
I wonder if there's something else in your environment that is causing the warping. Is the hanger touching only the middle of the mat? I can see this allowing the corners of the mat to turn down. The hanger I use has two clips - for my large mat, I hang it portrait style and the clips are at about 1/3 of the mat's width from each end.