Speaking as a packaging professional - thirty five years at UPS - I can assure you that Paper Hollywood nailed it with his post about double boxing.
To elaborate: the first thing you'll want to do is to find a carton that will fit your model, leaving a space of an inch or so on all four sides. Then you'll want a second carton in which the first will fit with the same one inch margin on all sides. These boxes should be new or like new, per UPS' gudelines; believe me, this will make a difference.
Start packing your your model by covering it with something like a plastic shopping bag. This will prevent having any loose bit of crud sticking to your model and also minor paint abrasions. Place a bed of whatever packing you're using on the bottom of the inner carton and set the bagged model on top of that. Add more cushioning material to fill the box; you want to put in enough to keep the model from shifting, without filling the carton with so much stuff that it will crush the model when you tape up the flaps.
Now of course, you tape up the flaps. Use decent packing tape - I find the Scotch brand lousy; it's very stiff, which prevents it from sticking as well as it should. But don't get cheesy dollar-store tape, either. Apply it in an "H" pattern with two pieces at either end, running at a 90 angle across the seam where the outer flaps meet and one or two pieces taped longitudinally along the seam. The first two pieces hold the flaps closed against each other, using the strength of the tape - its length - to the best advantage. The tapes running over the seam will keep the ends of the flaps from being pried up during handling.
Now label the box clearly, just as you will the outer box. You're ensuring that if anything should happen to the outer carton, the inner carton will still contain all the information the Package Service Clerks (of which I was one) will need to get your package to the consignee. BTW, make sure both cartons have no old labels on them; that outdated information could confuse the PSCs and delay your shipment.
Finally, repeat these steps with the outer carton. If you follow these methods you can ship almost anything without fear of damage in transit. Also, if you ship UPS, your model will automatically be insured for $100.00. If the model is of a greater Declared Value, you can buy insurance at $0.80 cents per extra hundred dollars of additional insurance; package tracking comes free with UPS.
I know it sounds like I'm still on the payroll at UPS, but I'm just giving you all the information I can. Maybe a FedEx or DHL employee will something have more to offer. I know the Post Orifice doesn't!