Biggest question is of course, how far are you shipping them? From where to where?
Regardless, though, here are some ways I've shipped large prints. If you want something ready-made, you can buy "Print Pads". These are hinged cardboard pieces that you attach the print to with corners and then this entire piece slips inside another cardboard sleeve. The corrugation on each of the pieces goes in different directions, so it is considerably sturdier. Also, you can pay extra and get the hinged cardboard pieces with "puncture guard" - which means that the interior piece is lined with a thin rigid plastic. I've shipped unmounted 18x24 prints to Sweden via FedEx with these. If you go this route, make certain to wrap the entire package in plastic to prevent water from penetrating the cardboard. And of course, make certain to fully seal all the seams with tape. No point in wrapping in plastic if there isn't a full seal. You probably can find the Print Pads at different places for different prices, but here is a link to one site. http://www.masterpak-usa.com/cat_102_printpad.htm
If you want a crazy strong DIY solution, you can just sandwich MDF or plywood panels around the prints and then screw them together (as long as the stack of prints isn't too thick - and I generally use a cardboard layer between the prints and the wood). This is basically creating a thin makeshift crate. Once the pieces are screwed together, you tape around all the edges to make certain water can't penetrate. I've shipped 20x30 unmounted prints to Lebanon via FedEx this way with no problems.
The above solutions have worked for me in sending delicate, valuable pieces internationally using carriers like FedEx, who really couldn't give a damn about the artwork inside - hence my making them rather indestructible.
If you are shipping continental US and want to avoid carriers like USPS/FedEx/UPS, you can always hire space on a fine arts shuttle. Since professionals would be handling your work at all times, you can simply sandwich the prints in cardboard and wrap that in plastic and send them on their way. (shuttle services are generally the cheapest service that fine art shippers offer, but it is still ridiculously good service)
You could always do price comparisons between lightweight packing and fine art shipping vs heavy duty packing and mail/package carrier services. If you want any recommendations for fine art shippers, let me know.
Hope the above helps.