Here I'd like to offer some advice for keeping your guitar healthy. It's pretty amazing how many great players don't know the most basic things about taking care of their instruments, and hopefully you'll pass some of this information on to them.
KEEPING YOUR GUITAR CLEAN:
Playing your guitar a lot can cause a nasty layer of gunk to build up on the strings and neck. Sweat, dirt, and skin from your fingers wears out the strings, and can also become a permanent stain on the body and neck if left too long.
The best way to minimize such built-up crud is to wash your hands before you play, and wipe the strings down between songs. I always have a soft, dry piece of cloth nearby, for both the strings and my sweaty fingers. If you're already careful about keeping your strings clean, you know how gross your guitar can get when you forget to wipe things down right away.
Another factor that contributes to guitar grime is string lube. Quite a few brands have popped up in the past few years, from sprays to lotions to oily clothes. These products claim to clean your strings and reduce friction so that your fingers can zip up and down the neck. While I agree that using such lubrication makes the strings pleasantly slippery, it's important to understand that such products will also collect along your frets and need to be cleaned off.
To clean your guitar, spray some guitar polish (Martin makes a nice one) onto a rag and then rub down the dirty spots on the body of the guitar-- front, back, and sides. Remember that grime can stain the guitar if it goes untreated.
You'll also notice a lot more crud where you tend to play more, usually around the first few frets, or maybe where your arm rests on the top or sides. It's a good idea to clean the neck when you change your strings, cleaning up and down the neck as you change each individual string. A filthy neck quickly dirties up the strings, causing them to lose their brightness and break easily. If the neck is really dirty, use a very soft bristle toothbrush gently along the frets until the neck is clean. Scrub up the gunk, but not very hard, or you'll knock the frets loose.
THE EVILS OF HUMIDITY AND DRYNESS:
Humidity and dryness can both really reek havoc on your guitar. If it's humid(over 60%), the last place you should keep your guitar is in it's case. That's where humidity collects, and your poor guitar will soak up the moisture like a sponge, and bubble out likes it's been overeating. This is particularly true of very high end guitars.
During the humid months, don't lock your guitar up in a small room with the windows closed, because the humidity and the oven-effect can really cause damage. By simply keeping the guitar in the open, and not in the closet or basement, it should get enough air circulation to keep it from soaking up too much moisture.
You'll start to notice the effects of humidity when the sides, back, or top begin to swell out(especially areas surrounding an inlays). Your action will probably get so high that the guitar becomes very difficult to play, sometimes frets even become loose. Worst of all, the sound quality of the guitar will begin to distort. Happily, most minor damage from humidity will repair itself once the humidity goes back to normal. Although sometimes there can be permanent scarring, most damage from moisture is purely cosmetic. Anything that requires adjusting can usually be handled be a competent repair person.
If you live somewhere with a really humid monsoon season and don't have a chance to get your instrument dry, then you can set it out in the sun for a short time like drying laundry. Be aware, though, that the sun will bleach the color, so be sure that your strap isn't draped across the front. Everyone likes and even tan.
The real guitar demon is dryness (below 40% at 24 degrees). While humidity is fairly easy to repair (depending of course on the extent of the damage), drying out your guitar for a long time can cause nasty, permanent damage. During dry months, be sure to have a humidifier (hardly expensive) somewhere in the room where your guitar is kept. Heaters in the winter time and summer air-conditioners can really dry out the air in the room, so you may wind up using your humidifier all year round-- not just for your guitar, but for your own health! As one guitarist told me, "Don't leave your instrument any where you wouldn't want to be yourself."
The effects of dryness include cracks around the braces on the sides and back, and the action will probably drop down so much that the strings start buzzing on the frets. Frets also tend to stick out, and your sixth string might begin to get snagged under the protruding frets. The worst problem occurs below 30% relative humidity, when excessive dryness causes the top to crack. Damage from dryness should be dealt with as soon as possible.
The effects from dryness and humidity are so easily controlled, that as long as you're not careless you shouldn't have any problems.
HOT AND COLD:
As the guitar is made of wood, it's also susceptible to temperature changes. Never leave your guitar in the trunk of your car during the summer-- built-up heat can easily clear 60 degrees, which will loosen the glue joints. Such a problem will not correct itself when the temperature drops back to normal, and would involve a major repair job to fix.
When you bring your guitar in from the cold, let it warm up in its case slowly, rather than popping it out into the extreme temperature change of a warm room. Leaving your guitar in the car during sub-zero weather for a short period of time isn't going to damage it like extreme heat can, but remember that cold months are often quite dry, and Jack Frost can really nip at your guitar if you give him the chance. There is also the problem of the strings contracting and tightening in cold weather, which can create abnormal tension on the neck.
VERY BAD GUITAR CARE:
In many guitar shops in Japan, the strings on the guitars have been loosened to avoid damaging the neck from long periods of string tension while the guitar awaits to be purchased. NEVER BUY THOSE GUITARS. I really can't express how foolish that type of "guitar care" is. Guitars are made of wood, and they are designed to have a constant amount of tension on the neck. Changing the tension will cause the neck to re-adjust itself to the new tension. If you buy a guitar with the strings loose, then tighten them up when you take the guitar home, your new guitar is going to adjust itself, and the play-ability will be significantly different from when you tried it in the store. This is particularly true for high-end guitars like Martin, Gibson, Lowden, Larrivee, and those made by private luthiers.
On top of the string-loosening thing, many music shops in Japan point bright, hot lights at the poor guitars to make them look attractive to potential buyers. This of course, just dries out the wood. Many stores are hip to the dryness damage from using high voltage lights, so to compensate, they pump, and I mean PUMP, humidifiers into the room like the place is a green house or something. More than a few times I've seen so much steam belching out onto rows of sad, helpless guitars that a film of moisture collected onto the strings and body. Too fight the moronic effects of hot lighting, these shops have gone to the other extreme. On top of all that, if the strings have been loosened, and usually they have, then I suggest the following equation:
dry heat + tropical humidifiers + abnormal tension =
DON'T BUY THE GUITAR.
When I see this in the guitar shops in Tokyo, I usually pull the manager aside and start up a lecture about guitar care. Hopefully this will increase your awareness.
THROUGH THE MAIL OR ON THE PLANE:
Folksinger Tom Paxton says it best with his song, "Thanks American Airlines for Breaking the Neck on My Guitar". Most airlines tell you that you're not allowed to take your guitar on the airplane because it's too big. On top of that, they offer no insurance in case anything gets broken. But rest assured, you don't have to pass your guitar onto the baggage check gorillas-- I've flown close to a hundred times, and only had minimal trouble getting the guitar on board. The best thing to do is not ask anyone if it's all right to carry your beloved guitar on, people enjoy saying "no" far too much.
If for some reason you feel compelled to play baggage-roulette with your guitar, loosen the strings just a bit, about a whole step and a half. It gets way below freezing in the baggage compartment, and with huge changes in air pressure, too much or sudden tension on the neck from extremely tight strings can cause it to snap (remember, cold causes things to contract). But as mentioned earlier, guitars are designed to have constant tension, and completely loosening all your strings just invites trouble. The peghead with the machine heads and the neck are by far the heaviest part of the guitar, and without tension from the strings to balance that weight off, one good jar regardless of careful packing could crack the neck. At least give your guitar a fighting chance by keeping close-to-normal amounts of tension on the neck. The same goes for sending it through the mail.
GETTING YOUR GUITAR ON THE PLANE
When you check in your other baggage, leave your guitar over in the corner where you can see it, but where the check-in people don't think it's part of your luggage.
After checking in, go get your guitar. You may be questioned about the size of your instrument at X-ray during your trip down to the flight gate, but that's pretty rare. Just nicely explain to them that at baggage check-in you were told that it would be fine to take it on the plane, and that you'll double check with the flight attendants when you board. I also find that carrying the guitar behind me with a shoulder strap helps to keep it from being noticed.
When you arrive at the flight gate, don't sit with the guitar where all the flight attendants can easily see it, as it just invites them to give you a hard time about it being too big for carry-on.
When you board, make sure that you're one of the last people to get on. Airplanes are on a tight schedule, and no one will want to hold up the flight just to go put your guitar down with the other baggage (although they may threaten to do so). Again, if you just carry the guitar behind you, most people won't even notice that you have it.
A couple times stewardesses have insisted that if I take the guitar on the plane, that it must go into the closet in the back of the plane. This worked out fine, because my case has a nifty little hook for a coat rack.
Once you're on the plane, you may have the problem of where to put your guitar if it doesn't fit into the overhead compartments. You can sometimes store it safely on an empty seat, under the seats, or really politely ask a flight attendant to put the guitar in the closet for you. After landing, they'll be happy to get it for you.
I'm always very nice to everyone, and if there's a problem, I explain that the people at baggage check said it would be fine if I brought my guitar on the plane, and I'm really worried about my very valuable collector's edition guitar, and isn't it almost time for take-off?
I've flown at least fifty times internationally on some really strict airlines, and very rarely have I ever been questioned about my guitar (although almost every airline has a no-guitar-carry-on rule). I have never once had to abandon my guitar to the fate of the baggage-check. I encourage anyone who cares about their instrument to do the same.