Unless your acoustic guitar is one of the few constructed from graphite, resins, and metal, it is made from wood. And wood is an organic substance subject to changes in temperature and humidity. You need to treat it with the same care that you would bestow on other living entities in your possession-say, a pet, a houseplant, or even your own body. Hey, that's no joke. Right off the bat, the best rule of thumb is that your acoustic guitar should never be kept in an environment that you would not be comfortable living in yourself.
Dry Heat: The #1 Enemy
Apartment dwellers and homeowners with electric heat need to be the most vigilant where humidity or the lack thereof is concerned. An excess of or a lack of sufficient humidity will have a most detrimental effect on condition of the body and the sound of the instrument. Too much and your guitar essentially becomes saturated, and it will thud instead of sing when the strings are struck. The wood, absorbing moisture, can expand to the point where stress and strain is put on the joints and seams. Be aware, however, that it would take considerably more moisture than exists in most households (outside of Calcutta, anyway) to cause those problems.
No, by far the more serious and more common condition is too dry of an indoor climate in the winter. This will result, at the very least, in your guitar losing its resonance and sustain. At the worst, the finish will crack or "craze" like the varnish on an Old Master's painting, the top and back seams may separate, and the guitar's soundboard actually splits.
The Answer: A Humidifier
One solution for the humidity problem is a room humidifier. Both you and your guitar will breathe easier. If you tend to leave your guitars out all the time, on stands or hanging on walls (which looks cool but is generally not a great idea), the room humidifier is a must. They're available at household and hardware stores, and some of them cost under fifty bucks. Make sure to have a hygrometer around to check the percentage of humidity. Cheap hygrometers can be purchased at most hardware stores and will work fine. A comfortable level for you and your loved one is between 40% and 70%.
If the room humidifier doesn't make sense for you, an excellent solution is a guitar humidifier that goes right inside the soundhole of your flat top or in the f-hole of your arch top. The most effective guitar humidifiers consist of a rubber tube with a sponge inside and have a basic "paper hygrometer" attached to the round, plastic disk that seals the soundhole. They are cheap, simple to insert and remove, and effective. Just follow the easy instructions and you will be protecting your investment.
Check That Neck
Besides its effects on your guitar's body, humidity will affect the neck as well. Most guitars, whether electric or acoustic, need some bow or "relief" in the neck to play without buzzing. The relief can be checked easily. Hold your guitar in a seated position as if you were going to play it. Place your left hand index finger on the low E string right on fret 1 (not in between frets, as if you were going to play an F note). Then, while holding the guitar against your body with your right arm, place your right hand index finger on the fret where the neck joins the body. On most non-cutaway flat tops this will be at fret 12 or 14. You have now created a straight edge, or ruler, out of the low E string. Look at the area around fret 6 or 7. There should be a tiny space approximately the thickness of one or two business cards between the fret wire and the bottom of the low E string. If it is much more, or if the string touches frets 6 or 7, you need to adjust your truss rod. (See "Be Truss-worthy" in Time For A Tune-Up). Follow the procedure for your particular instrument, or, if you are not comfortable doing this yourself, take it to your local guitar shop or repairperson.
One last tip: A good, non-abrasive polish like Martin Guitar Polish will help to protect the finish on your delicate friend. Other quality polishes made expressly for the guitar will also work, but stay away from spray-on furniture polishes found in your local supermarket.