When I shipped my wheels and tires, I just used the round circle cardboard that you find inside wheel boxes (when wheels get shipped, they're used on the inside to further cushion them) and taped them onto the wheel/tire combo. Then shipped them just like that. Use a lot of tape. The tire will help protect the wheel, as does the cardboard. Remember, they'll be stacking your wheels along side of other cardboard boxes, so its not like you have to protect them from rocks or the like.
Very simple and effective.
