Sculpting Sculpts/Core Issues

I've been working with clay for a little while now trying to get my cores to actually work for me. Unfortunately all of the cores I've tried seem to only stick to my clay and I can never remove them properly without the destruction of my doll.

:'( It's a lot of practice in actually sculpting something, but I'm getting annoyed with the fact that I can never continue with the process. I really want to finish of of these dolls, even if it's just the sculpt.

I want to use the core method, but I don't know where I'm going wrong. For the core I've tried styrofoam wrapped, and regular aluminum foil. The sterofoam works all right, but it's messy to shape and I'm not sure I've gotten the right stuff. It's also annoying to realize that I've gotten plastic stuck in my clay.

Anyone have any suggestions?