What's the best way to make a mold from a clay sculpture?

I love sculpting on self-hardening clay; most of my sculpts are irregular pieces like busts and full figures(mostly fantasy figs). I found out though that the clay is fragile and a bit too heavy. I would like to cast them in plaster of paris but I can't think of a good medium. Also, will the "foam" dentists use to make a cast of teeth work?