1) Loosen the strings or you risk snapping the neck
2) Put padding in the case so the guitar does not 'rattle around' at all
3) Take some tape with you (clear packing tape) to ensure the clasps do not come undone and your guitar then fall out onto a hard floor
4) Open it immediately upon arrival, and if there is any damage, report it before you leave the airport
5) Insure it for the replacement value of the guitar AND case
6) Clearly mark it with the tags they have at the airport
7) Have the airline put lotsa "FRAGILE" stickers on it
8) Put your name underneath the case where it would not be normally seen by anyone in the event there is a dispute as to whose guitar it really is
9) Keep a record of the serial number with you