A metallic silver or gold sharpie for making additional marks on the foam. Also, order extra cutting pads. Always have an unused one on hand. It's a consumable item, like a rotary blade and you don't want to be caught without one on a Sunday night.
A good pair of tweezers. The cutter will come with a pick to keep the dies free from threads, but sometimes you can get them wedged where the blades join.