Classical and flamenco guitars tend to be more susceptible to improper humidity than their steel-string and electric counterparts. With a little bit of knowledge and common sense a guitar can be kept in good shape, preserving not only its playability, but its aesthetic beauty and value as well.
The worst threats to a guitar's 'good health' are: excessive dryness, excessive humidity, and sudden changes in temperature/humidity. However, it cannot be emphasized enough that excessive dryness is the most dangerous of the three.
Excessive dryness is a condition that can be caused by natural atmospheric conditions. However, living in a region with high humidity does not necessarily mean the guitar will not be exposed to dryness, because both air conditioning and heating systems can dry out the air.
Most high-quality classical and flamenco guitars are built in workshops where the relative humidity is maintained at approximately 50%. It is therefore recommended that your guitar be stored at this level. Storing your guitar in 40%-70% humidity is generally safe. But anything below 40% can potentially be dangerous. If humidity falls below 40% it is very common for the fingerboard to shrink, causing the frets stick out. If humidity drops below 30% warping and cracking could occur. Depending on the extent of the damage these problems can usually be fixed. If dryness is detected early enough, moving the guitar to a more humid environment will solve the problem. On the other hand, if damage is too advanced the frets may need to be filed down and/or the cracks repaired. The worst case scenario is when the damage is so advanced that the structural integrity of the instrument is compromised. At this point the best option is to take the instrument to an expert repair person to determine whether the instrument is salvageable.
As with all problems, prevention is the best medicine. There are two simple ways of preventing this type of damage from ever occurring. The first is using a humidifier in the room/house the guitar will be kept in. Humidifiers of this kind are sold by most department stores and they come in all shapes and sizes. The second, less intrusive method, is to humidify the case where the guitar will normally be stored. This not only is easier to monitor, but it is also more practical for people who travel with their instrument. If this is your method of choice the Humicase is a great option: it consists of a case with 2 built-in humidifiers, strategically placed to distribute humidity evenly throughout the interior and maintain it at the optimum level.
If you already own a good case you might consider using a case humidifier such as the Credo Precision 70. Credo is known for manufacturing high-quality Cigar humidors and humidifiers. Their product for musical instrument cases works with a humidifier solution; this special liquid, together with the humidifier, helps regulate the humidity level much better than if you were to simply use a sponge with water inside your case. Do be careful to make sure the humidifier does not directly touch the guitar: direct contact with water could damage the wood/finish, or the humidifier itself could scratch the finish. The Credo comes with a special adhesive so you can attach it permanently to your case and avoid this problem.
Regardless of the type of humidification you employ, you should also consider a good dial or digital hygrometer to monitor humidity levels. GSI offers a Humidity Control Package, which includes the Credo Precision 70 Humidifier, the Credo Humidifier Solution, a Digital Hygrometer and a bottle of D'Addario/PlanetWaves guitar polish.
As stated earlier, two other conditions to avoid are excessive humidity and sudden changes in temperature/humidity. Damage from either of these circumstances is much less common, and usually less severe. The most common problem associated with excessive humidity is a slight loss in volume and quality of tone. This can be easily remedied by bringing the instrument back to the appropriate level of humidity. But, DO NOT TRY TO DO THIS QUICKLY OR OVERCOMPENSATE. Some people have tried to remedy this loss of volume/tone by 'drying out' the guitar in direct sunlight, only to pay a steep price for the sudden change in temperature and humidity the guitar experiences.
Simply put, avoid any extremes and use your common sense. Do not leave your guitar for long periods of time in direct sunlight; avoid car trunks (which can fluctuate from very hot to very cold quite quickly); do not hang your guitar on a wall (walls have much lower temperatures than the air around them), especially during winter months in cold areas; do not set the guitar next to sources of heat, etc. Very important: if your guitar is exposed to freezing temperatures, do not open the case immediately after going into a heated room! Let the guitar warm up to room temperature while it is still inside the case.
All this may sound like a great deal of trouble for a guitar, but given the size of your investment and the agony of discovering a crack in your fine instrument, we believe that a bit of effort in maintenance will be very much worth the time. If you follow these steps and take good care of your instrument, you should get a lifetime of enjoyment from your guitar.