I've used a lot of cardboard for sheet mulching but typically with mulch or woodchips over it. Have you heard about the back-to-eden garden method? If not check it out. I have also used old jeans as sheet mulch. The denim (or twill after I wear out khakis) works well and lasts a little longer than the corrugated.
Another cool idea I've seen is built in mulch using a pallet. Put the pallet on the ground and fill it with soil then plant between the slats. I think you should cover the open ends but I've seen people not have them covered. The slats will act like mulch but also keep your plants spaced and in nice neat rows. Typically I've seen this with smaller plants like greens and beans.
