After the plaster has set and is no longer warm, use several flat screwdrivers or similar flat wedge objects in several places around the shim line and begin to pry open the sculpture. This may take a lot of work, especially if there are several undercuts. The faster you do this process, the warmer the clay will be and hence easier to pull apart. If you leave the clay inside the cast for longer than a few days, the cast will be very hard to pull apart. Paint on two coats of Murphy's oil soap to act as the mold release.
