Whenever I've used the white, crumbly packing case styrofoam I've lined with plastic or considered it a one time mold. If it's the crumbly kind of styrofoam you could try lining and taping down dry cleaner bags. Then as soon as you unmold, about 24 hours, use a scotch brite pad to smooth out the lines left from the dry cleaner bags. I've done that to remove the lines from planters.
If this styrofoam is the very dense, smooth, stuff like ridgid insulation sheets no release or lining is needed. I use the ridgid insulation for trough molds and you can reuse them over and over and the sides don't stick to the hypertufa. Billie

Billie: Thank you so much for your speedy reply! This is the rigid, high density foam. I have several of the forms so I think I'll try it both ways & see which works the best.

Also thanks for the great tip on using a scotch brite pad on the wrinkle lines!

syllabus, Just make sure you unmold soon enough and handle the piece gently. Most people wait too long to unmold and the piece starts to "hard" cure. Just take care because you can crack it or break edges. Keep an exact record of the recipe/mix that you use in case you have to make repairs. When mixing up a batch of cement and whatever, a friend of mine, saves some of the mix before adding water.

I WONDERED ABOUT USING THE STRYOFOAM BEADS FROM BEAN BAG CHAIRS INSTEAD OF PERLITE. ANY THOUGHTS?

The perlite and vermiculite are minerals by origin, they act like aggregate in the setting process. The styroform can't substitute them. You might can use it instead the peat moss, but then you have to use sand too in the mixture. I have no experience with it. Try it with a batch.