Cardboard box works just fine. I boxed mine up 5 times this year in various boxes and had no issues problems whatsoever. The boxes are usually free from an LBS. I grab one from an LBS, fold it up as much as possible, shove it my pannier and cycle to the airport/train station where I do the disassembly game. Get the biggest box you can get ahold of too, box sizes do vary quite a bit.

Best tip is throw all your gear inside the bike box. Saves you a piece of luggage to check in and protects you bike