I use 1/2 inch conduit as a rod (its cheap, 10 feet is a little more than $1), then I buy coated S hooks off fleecebay. I bend the top section 90 degrees, so that the wheels run perpendicular to the rod. I use nylon zip ties to keep matching sets together, then hang the pair from one hook. Finally, I mark orphan wheels with fluorescent survey tape so they stand out.
Did I mention I have a lot of wheels??
Something similar to these:
http://www.ebay.com/itm/NEW-Plastic-Coated-Steel-Core-S-Hooks-50-RED-S-Hooks-/200848733363?pt=LH_DefaultDomain_0&hash=item2ec38474b3
http://www.ebay.com/itm/Powder-Coati...item4d0362df12