craftgrrl | Shrink plastic and warping

Hi there, not sure if this community is really open to requests but I'm having a little trouble with a craft project I'm working on.

I'm trying to make some designs on shrink plastic, I believe in America the proular trade name is Shrinky dinks?

During the heating process I'm finding that mine tend to curl up and stick to themselves, and even if I pull them out of the oven, press them flat and reheat them, they never quite lay flat again, like they developed a kink.

Does anyone know of a way to keep them fairly horizontal and straight during the heating process?