I have been using one of those electric knives - the kind that you use to cut meat. I found that I get more control if I make sure to have a part of the foam that I am not cutting resting on the table. It doesn't wiggle around so much and I get a better line. I have used the other type - the one that melts with battery power - on styrofoam and when making props out of blue foam (reticulated foam?). I never have tried it on puppet foam though.