If you are flying with your guitar, there are a few things to keep in mind.
First, the FAA has told the airlines that musical instruments (within reason, so no kettle drums here, but guitars are specifically included) are to be considered as allowed additional carry on luggage. The airlines are allowed to ignore this, but if you make a stink about it, it usually works. Ask your local musicians union to get you a copy of the letter from the director of the FAA to the nation president of the musician's union.
Second, FAA regulations are very specific, the ONLY people who can say you MUST check you guitar are the pilot and the lead flight attendant (or at least, that used to be the case, and I assume it still is). If a baggage clerk or gate attendant says other wise, smile and nod, and then take your guitar on the plane. If they insist that you must check it, then get a hand carry tag at the gate, which allows you to walk down to the end of the jet way and have it carried from there. Put the tag on your guitar, and then ignore it. Walk on the plane with your guitar, and put it in the overhead compartment. Unless one of those two people tells you other wise, just smile and be polite, but ignore it. (Of course, I haven't had to fly with a guitar since 2001, so you should probably listen to an Air Marshal if they say something. They have guns, after all.)
Which brings us to number three on Light's Tips for Flying with Guitars (which I used to do at least 6 times a year). Get a seat which boards early. This means, if you can, get a seat in an exit row (if your airline boards exit rows early, as Northwest does, though I don't think AA does), or in the very back of the plane. You want to be one of the very fist people on the plane. Walk on with your guitar, put it in the overhead, and as soon as the overhead is full, close the door. I've never seen a flight attendant open one of those things yet when looking for more room. Don't be a prick to other passengers or anything, and let them put smaller bags in around your guitar, but as soon as it is full, close it.
By the way, if you are going to be flying in the back of the plane all the time, make sure you bring ear plugs. The engines get fucking loud back there.
If you DO have to check your guitar, you want it in a hard shell case, (a flight case if you can, such as a Calton), and it is a good idea to pack it up the same way you would if you were shipping it any other way (UPS or FedEx, etc.). Whatever you do (checked or carry on), MAKE SURE you COMPLETELY slack the strings. A blow which would not otherwise do any damage will frequently make the peghead snap right off if the strings are at tension. Also, you want your guitar hand carried if at all possible, because you don't even want me to get into the shit that airlines will do to guitars when they go through the normal checked baggage lines. I know of no single entity which completely totals guitars with the frequency of the airlines.
Most importantly, though, is that whatever you do, be polite and courteous at every step of the way. Remember, those ticket agents and gate attendants are dealing with working for companies who are doing everything they can to fuck their employees (just ask a former Northwest mechanic, who have been told that about 10% of them can come back to work, at a 70% pay cut, with the understanding that they would be supervised by the asshole scabs. A 25 year old scab is all of a sudden the boss of a guy who has been fixing planes longer than the new "boss" has been alive? Does anyone really think that is anything other than being fired?) They, understandably, are trying to exert authority which they don't really have. Just smile and make them think you are listening, and then do what you were going to do in the first place.
And make sure you put your string clippers in your checked baggage.
Light
"Cowards can never be moral."
M.K. Gandhi