I had to do this just recently to take my Half Life 2 HEV suit to Dragoncon.
If you're flying, the FIRST thing you should do is check the size limitations for checked baggage for the airline you're planning to fly on. For me, I was flying Delta, and their limitation is that the length+width+height of your bag has to add up to 62 inches or less. If your bag is bigger, they classify it as "oversize" and they charge you a LOT more!!!! (normal checked bag on delta is currently $25 for the first bag. If your bag is considered "oversized" that shoots up to $200!!).
What I ended up doing was getting a cardboard box from Office Depot, along with a couple bags of packing peanuts and some packing tape. The box was 24" x 18" x 18", which adds up to 60", or just 2" shy of Delta's limit. It was pretty ghetto, but it worked. If you go that route, make sure you tape your box up thoroughly and make sure there are no small, loose items that could fall out of a hole in the box. On my trip to Atlanta, my box arrived with about a fist sized hole in the side. Not to serious, but if I'd had lose small items it could have been bad. On the way back I completely circled the box with packing tape and that seemed to make it a lot sturdier. It arrived with me in Denver in exactly the condition it'd left Atlanta in. Also make sure your name is on the box in at least one place, and that it's marked in some fashion so that nobody mistakes it for a non-luggage box!
A hard side case or something like that would definitely be far superior and I plan to look for one between now and next years DragonCon.
However, far more important that what you pack your armor in is HOW YOU BUILT YOUR ARMOR!!! Don't make the mistake I did and not even think about how to pack your armor until the day before the flight! Think ahead and try to make sure your armor can disassemble into manageable pieces that'll fit whatever kind of box or luggage you have available. That'll save you a ton of headaches!