The single most important thing you can do to protect your guitar is to learn about humidity control. As a custom guitar maker and authorized repairman for many guitar manufacturers, I have all too often seen the serious damage caused by lack of moisture in the home.
Most acoustic guitars are built in a relative humidity (RH) range of 45%-50%. This range is right in the middle of the average yearly climate swing in countries where guitar is most popular. The danger to your guitar occurs during the fall, winter, and early spring heating seasons. The colder it is outside, and the more you need to heat your home, the drier the air becomes in your home. It can often drop to less than half of the RH of the factory.
When the RH in your home drops to 35% or lower, look out! The guitars top will start to sink in, the neck angle can change, cracks often occur in the top, the bridge can lift from the top, and action will drop, causing fret buzz. Most guitar manufacturers will not honor warranties if you allow your guitar to dry out like this, and the resulting repairs can be very expensive. And in the guitar body humidity control device like the Planet Waves Acoustic Guitar Humidifier is essential if you want your guitar to be safe and healthy.
This device will protect your guitar even in the driest environments, provided that you keep it adequately filled with water. The thing I like most about the Planet Waves humidifier is that you can't overfill it. Once a week, just squeeze in water with the included syringe (over the sink) and excess water will vent out of the unit. Wipe it dry and you're all set! I can't stress enough how important this is. Individual homes and regions will vary, but check your Planet Waves Humidifier often to keep it moist!
For those of you who wish to really be on top of this issue, a humidity meter called a hygrometer is available in many stores, catalogs and guitar supply companies. These range from $15 to $150, but even a simple one will tell you the RH fairly accurately. Planet Waves offers a combination kit that pairs the humidifier with their new digital Humidity/Temperature Sensor.
I tell my customers their guitars are happiest above 40% RH. Less expensive, laminated (plywood) guitars are less prone to severe damage from dryness, but still benefit from a humidifier for overall health, action, and tone.
You should not use a humidity device in the spring and summer when the heat is off, unless your home is very dry due to desert climate or extreme air conditioning. Though some of your guitars may swell a bit in the humid summer months, it is temporary and all will be fine. However, if your guitar is left in an environment that is too dry, you could be in trouble, and the scars from the resulting damage won't go away.
Now go make some great music and protect those guitars!