For an acoustic guitar that you really truly care about, there are two small accessories you need to get that are absolutely worth having—a humidifier and a hygrometer. Your guitar will thank you and reward you with greater health and longevity (for the guitar, that is).
Small and inexpensive (about $12 on up), acoustic guitar humidifiers come in a few different types that go either in your guitar’s case or in the guitar itself. These include the clay-filled type about the size of a film canister (remember film?), the soundhole-cover type and the rubber-tube type, each of which works basically as a saturated enclosed sponge that moisturizes the wood without actually dripping water anywhere on the guitar (hydrating your guitar is good; actually dripping water on it is bad).
A humidifier is a good idea because guitars are made of wood, and wood reacts to changes in humidity and temperature. Really good acoustic guitars are built in humidity-and temperature-controlled manufacturing environments, and maintaining the recommended moisture and temperature levels present during your guitar’s construction will help prevent problems such as fret buzz, warping, cracking, top sinking, etc.
What is the optimal humidity level for your acoustic guitar? About 45 to 50 percent.
Put another way, your guitar is comfortable in the same humidity and temperature range that you’re comfortable in—neither of you comfortably withstands prolonged exposure to extremes of heat, cold and dryness. A good acoustic guitar is built to tolerate a certain amount of gradual climate change, but sudden big changes in humidity temperature can really cause trouble—like leaving a guitar in a hot car and then opening the case in an air-conditioned room, or bringing a guitar that has been sitting outside in the cold into a heated (and likely very dry) room.
This is where your case comes in too, by the way. Besides getting your acoustic guitar from point A to point B, its case also helps protect your guitar from heat, cold and, with the help of a small humidifier, dryness. Keep your acoustic guitar in its case when you’re not playing it, especially if you live in a particularly dry climate. When moving it from cold to warm environments (or warm to cold, dry to damp, damp to dry, etc.), don’t open the case right away—let your guitar acclimate to a different environment for a short period while it’s still in the case.
And if you get a guitar humidifier (and you should), you ought to get a hygrometer to go with it. A hygrometer is a small device that measures humidity. Put it in the room where you usually keep your acoustic guitar or, even better, right in your guitar case (fastened by Velcro to the outside of the accessories compartment, facing the heel of the guitar is a good spot). Really good ones cost $30 to $50, but there are plenty of less expensive ones that work just fine, and they often come packaged together with humidifiers as complete humidity control sets.