Hi there Nathan,
I use a single edge razor to cut out foam. A sharp blade cuts through the foam nicely. You can also use the blade to put a bevel on the edges of your foam pieces, to soften/round them a bit.
Just watch your fingers! Sometimes, if I am going to be doing a lot of cutting with the razor, I will wrap some tape or Band-Aids around my thumb, pointer, and middle fingers. This stops the cuts before they happen.
Hope this helps!