If you plan to check your acoustic guitar at the airport as baggage, there are several precautionary steps you can take to protect your instrument.
First of all, pad the peghead. One of the most common injuries to a guitar is a broken peghead. Pack extra materials on all sides or buy a case that has extra peghead support. Secondly, release the tension in the strings. On some acoustic guitars, the strings can hold as much as 150 lbs of pressure---slacken the strings to make sure they and your instrument can withstand a bit of jostling.
Most importantly, look for a sturdy travel case. The stronger the better---a hard-shell case is a good investment, especially if your guitar is worth a lot. Make sure, however, the case isn't too heavy. It does no good to have your guitar land safely if you can't carry it it out of the airport! What's more, airlines these days have strict weight requirements for checked bags; if your case is too heavy it may cost you extra to check it.
If you have latches on your case, secure them with duct tape. Case covers are also available if you are concerned about the residue left by taping your case. Lastly, go directly to the baggage claim area upon landing. The longer your guitar sits, the more likely it is someone will walk off with it. Note that sometimes instruments get classified as odd-sized or oversized items and will be brought out by an attendant by hand instead of being sent down the baggage carousel.
For some, the thought of losing or damaging their prized possession is just too much to bear. If that's the case, you may consider carrying your guitar onboard with you.
Check with the airline to verify they allow acoustic guitars as carry-on luggage. Some, like America West, US Air, Continental and Delta, have strict check-only policies, but many airlines allow instruments to be "gate checked," which means get a special claim tag and hand the instrument over to the ticket-takers as you board the plane. Your instrument is then stowed in a safer space in the aircraft along with items like strollers and wheelchairs; you retrieve your guitar as soon as you deplane---it isn't sent to the baggage carousel. If you have a small guitar, like a mandolin, you may be able to fit it in an overhead compartment, but considering how overstuffed those compartments get, you'll probably be better off gate-checking your instrument.
It's a good idea to use a gig bag that is not too bulky. Although you'll still want a little protection, the extra necessary padding might make the difference in whether or not your instrument is accepted as a gate-checked item.
