I either use the floor (not very often because cat hair never seems to disappear) or the toy box (now my hope chest) that my dad made me. It's only 18 inches high, so I have a small cutting mat on top of it and sit on one end and cut at the other. I've gotten quite good at folding things accurately for cutting. All bigger things get cut on the floor.
I've noticed that my back hurts more than my shoulders though and have started using a body-rite posture pleaser. You can google it, it's amazing for back aches when you just want to keep cutting hehe...