Jason from Auburn, Ontario has found a great way to cut rigid foam insulation.
Regular knives are too thick and get stuck in the foam. He sharpens the edge of a putty knife which is strong not by being thick but by being wide. Jason recommends sharpening only one side of the knife to cut better but I don't know if that has much to do with its good performance for him. Professional cutting knives are sharpened on both sides. The real key is having as thin as possible a knife, and Jason's putty knife does that very well.
Another tip in cutting both foam and drywall, is to draw a line, reach way out in front of you and draw back with your whole body, keeping your arm straight. That will give you a much straighter cut than if you pull the knife past you, bending your elbow.