For cutting foam I use several different things. A shop knife with retractable blades for shallow cuts, and one of those snap-knives for deeper cuts. But recently, I found a knife in our kitchen that someone gave us as a gift, I think it's like the ones on TV where they'll cut a rock in half, then thin slice a rotten tomato after doing that.
Anyway, it has a surrated edge and works just great for rough shaping the foam. I've got a wood rasp and a wire brush, but they just don't cut it (real pun here)
, since I do all my final shaping using plaster-soaked paper towels and finish up using joint compound and various putty knives and other tools to get the desired texture and shape.