Since guitars are made of wood (mostly), checking and maintaining proper humidity levels is crucial in preserving the quality of your instrument. Because both air conditioners and heating units tend to dry out the surrounding air, sustaining proper humidity levels becomes even more crucial. If possible, try to keep the storage area for your instruments at around 45% humidity. Whole-room humidifiers will ensure that a music or practice room stays at a pre-determined level (Kenmore humidifiers from Sears are great for this purpose). If it's not feasible to humidify the entire room, then a portable humidifier is needed.

There are various types of humidifiers available, some more appropriate for acoustic guitars than electric. FYI, while it is true that acoustic guitars are a bit more susceptible to low humidity, both acoustic and electric guitars should be properly humidified. Some humidifiers simple rest in the case while others are actually suspended in the soundhole of an acoustic guitar. Be careful not to over-humidify your instruments, though, since that brings up a host of issues as well. Symptoms of low humidity include protruding frets and a cracked fretboard or top in extreme cases. The guitar may also become more difficult to play as the wood literally dries out. Fortunately, such issues are easily fixed with proper humidification.