you can have ups pack them for you... they are cheap... or you could wrap them in blue tarps vary tightly and lots of tape... that works as well. in my days at ups and dhl i think the wheels and tires in the tarps were treated the best and always looked to be in the best shape...
It depends on the size. When I ship them, I go to a local rim/tire place and get their throwaway boxes. I tape 2 of the boxes together with strapped packing tape.
I have also used the 18 x 18 x 18 U-Haul boxes and put 2 rims per box, but I double or triple-line the boxes with cardboard, and I put extra sheets of cardboard between them.