I have just received my new Cricut cake and like many others was just about to use it as a paper weight when I finally found a system that worked. so I thought I would share it for all of you who are still at the "throw it out the window" stage.
I used Room 4 cake Gumpaste.
I used my pasta roller to roll it very even to mark 5 so paper thin.
I then dusted it with a 50/50 mix of powdered sugar and cornflour on both sides.
I greased my mat so it was slippery but not too thick.
I laid the gumpaste on it and smoothed it well with my rolling pin.
I carefully trimmed the edges into the cutable area.
I then put it in the freezer for 30mins so the shortening got hard and stuck the gumpaste firmly to the mat.
after the time I immediately put the mat into the machine and made the selections and cut.
this works great. perfectly smooth shapes.
I used the pressure and speed on medium.
I was having to work quickly because it is over 30 degrees C here at the moment and the shortening was melting really fast.
but it did work well when cool.

hope this helps.

Thanks for the tips! I have one just sitting around mostly because I haven't needed it/figured it out, i'll have to do some more experimenting.

Could you please tell me what room 4 gumpaste is?

I have also had really good success using the edible image papers its slightly more expensive but no rolling, freezing or stressing. I will keep your tips handy though for when I need to work with gumpaste.