When I bought a sheet of the 3/4" white stuff, I spent a couple of minutes to pick the one from the stack that had all the corners and no cracks. Happy with my purchase I walked outside with it and the wind caught it, broke it into 4 pieces. Lots of folk got a good laugh as I went running accross the parking lot chasing the two biggest pieces in their quest for freedom.
So another rule for working with styrofoam--wear running shoes when you buy it!