Spending the afternoon shaping blue Styrofoam for part of our car. Its one of the worst jobs. Using handsaw, slim carving knife, random orbital sanders (80 and 150 grit, and connected to shop vac), its a messy business. The Styrofoam chips and dust floats then sticks everywhere. Having a shop vac connected to the sanders is a definite requirement.
Another thing I hate about styrofoam is how it seems to take a “set” when sanding. Hand sanding in one direction sets up the foam cell structure to rip when the sanding direction is changed. This often happens when you are doing the final touch ups.
Grrr.
As I carve away, I am thinking bad thoughts about those fortunate teams that have access to a 3 axis CNC router – you know who you are.
canary notes 
I remember using an electric carving knife was just the ticket to slice neatly through urethane foam without undue crumbling – perhaps it would be appropriate for styrofoam as well.
I heard that also, and urethane foam I think is more brittle than styrofoam, in a good way. Sands and machines better. But don’t use hot wire cutter on urethane foam!