I always use folded silkspan (I have a lifetime supply of the Sig stuff). The exact, smartass answer, is to wet it and stick it to a glass plate, then let it dry. It'll be flat when dried. Then you can wet it (it'll stretch out to right where it was the last time it was wet) and apply it.
But really -- the creases go away as soon as you wet it. In fact, when I'm doing a large panel like a wing, I'll cut my piece oversized, crumple it up, dunk it in water, wring it out, then un-crumple it and apply it. There will be super-mild creases from my crumpling, but as soon as you start stretching it out over whatever you're putting it on, those will go away. Any that don't will go away when it dries.