I wanted to reupholster some dining room chairs on a budget. One of the issues I ran into was getting the right upholstery foam cutter and getting one at a reasonable price (especially since I do not plan on cutting foam very often).
That said, I picked up a Hamilton Beach Electric Knife, model 74378R, which is intended for carving and slicing meat, bread, and other food products. It was only around $10 at Walmart, so I gave it a try.
When I cut the foam with it, I held the cutter upside with 2 hands and made sure the blade was as straight as possible, this ensures the cuts would not be at an angle.
After cutting, high grade, 2 inch upholstery foam, I was impressed about how accurate and effortless the cuts were.
The key is to apply gentle, even pressure while you are cutting, there is no need to go up or down, just simply push forward on your cuts. Additionally, make sure the foam is stationary with some sort of weight on it to prevent it from moving.