This is confusing in itself. The form of your doll is built over something. Pros generally build over styrofoam. There are several kinds though.
For anyone who has ever been confused about this, it is not the white kind that has the little balls in it. It is the kind that you push fake flower stems into. It has a sparkle to it and looks sort of crystallized.
I have a rasp which has four settings. experiment with them to see what you like best
Regular styrafoam though is not without merit. It is a little more difficult to sculpt, but if you are patient you can definately shape it with a rasp. I had the most success if i used the very rough rasp, and used it with the grain. (Not against which would remove the most material)
I am doing an experiment where I cover each styrofoam with slip clay then will fire it to find out how it works. Pictures will be available once finished.