I've successfully shipped both large and small models all over the world. If you want it to arrive safely, pack it like it's your most valuable possession.
First, surround the model on all sides with at least 2 inches of convoluted (aka "egg crate") polyurethane foam (like the stuff in cases made for expensive cameras and other custom gear). Make sure the model can't move around.
Next, build a strong and rigid crate around it. You can make very nice crates using 3/4 square lumber and cheap 5mm plywood "underlayment" from Home Depot. It's the cheapest plywood I have ever found, lightweight, strong, and perfect for making crates.
Finally, pack the crate inside a cardboard box and secure well with packing tape.
TIPS: If the model has delicate spikey bits that will break if touched -- even by soft foam -- you'll need to get very creative in finding ways to secure it and keep if from moving inside the crate. Place foam wherever you can and try to pack it so the model doesn't move and nothing can touch the most fragile parts.
Most important! There should never be ANY direct contact between the crate and the model. For example, if the model is mounted on some sort of stand, make sure that stand is completely isolated from the crate.