shrinky dink help

[http://users.livejournal.com/_pakalolo_/]

so ive been trying to use inkjet printer shrinky dinks but for some reason, they just wont work for me. when i try to bake them, they dont lay flat and come out with little holes in them or dents or the edges end up rippled. am i doing something wrong? i put them in the toaster oven at 275 degrees until they get small. any suggestions?