Any creases caused at the folds will break down the material at the crease. You can store them flat with nothing on top of it or hang them with a wide (preferably padded) hanger so they take less space.
My preference is to hang them with some of the commercially available wide wetsuit hangers, but I have padded the hangers with some close cell foam pipe insulation. You can find the pipe insulation in any hardware store.
The important thing is to avoid creases and to store them in a dry, cool, clean place, out of any sun light. Preferably with some ventilation to make sure they are dry. I would never leave them in a bag folded.