There were directions for making a quick tire sled recently on the CrossFit boards. I made one in about 10 minutes with an old Goodyear. Basically cut a few 2x4's to span the interior, screwed in a piece of threaded, galvanized 1" pipe attached with a floor plate thing (technical term) to put weights on. Drilled a hole for an eye-bolt, which I ran through a small peice of 2x4 on the inside of the tire (so it won't pull through). And viola. Very simple, very effective.
We use an auto tow strap (3" webbing) to pull it, but I hear Cabela's has a fairly cheap harness used for dragging big game animals.