There was a thread on the CR list recently about the same topic. This si a summary of the consensus opinions:
1. Use a hard case. This generally gets loaded in the bottom of the baggage are with other bags above it. However, the case protects the bike.
2. Use the plastic solution as shown by Max. This generally gets loaded last and is generally placed on top of the bags, with nothing else on top. The plastic wrap seems to ensure the bike is not mis-handled. Someone stated Greg LeMond would wheel his bike into an airport, remove the pedals, turn the bar and get a plastic bag from the airline.
3. Do not use cardboard. Consensus is it offers little protection and the bike has a greater probabilty of being mishandled.
4. Call the airline and see if they allow a walk up (a la LeMond). Some may still allow it.

John Hawrylak
Woodstown NJ