Environmental Changes  

Temperature and humidity of the air around your guitar affect it more than anything else. Acoustic instruments are especially affected by dry air and extreme temperatures. Extended exposure to hot air, 80 degrees and above can loosen glue joints, braces and glue on bridges. Plastic cases can warp in direct sun sometimes exert pressure on guitar tops as well as create extreme temperatures.  

Dry air with relative humidity of less than 40% shrinks wood which causes stress on areas adjoining glue joints. The wood can flex and shrink and tear away from the glued areas. Softer moister wood will shrink and tear away from stiffer grained wood. Fretboards can dry out and shrink  causing sharp fret ends to be exposed, and frets to loosen and pop up. Also overtime fretboards can crack.  

Extended exposure to cold air can cause glue joints to become brittle and fracture. Laquer finishes which have chilled down will fracture (craze) if suddenly exposed to warm air. If you have been traveling in the winter and your guitar case is cold, let the instrument sit in the case till it feels closer to room temperature before you open it.  

Seasonal changes in temperature and humidity can affect the playability of your instrument. This causes high action in the humid times as the top swells, low action in the winter as the top shrinks. Necks can variously back bow in humid times and forward bow in times of dry air.  

Storing your Guitar  

If you plan to leave a guitar un-played for an extended period of time keep it in a case and loosen or remove the strings. A quality hard-shell case is essential for any guitar that you want to last. Keep the guitar in an area which will not be subject to extremes in temperature or humidity. If you do these simple things, your guitar will be ready to play when you are.  

Humidity  

The optimum relative humidity for guitars is 45-50%. Dry air kills and maims musical instruments so do whatever you can to maintain a stable relative humidity in the place you store your guitar. So if you live in an area of cold dry winters or hot dry summers you should have a humidifier to regulate seasonal humidity levels.  

Strings  

The most significant thing you can do to maintain the tone, volume, intonation, playability, and pleasure of your guitar is to change the strings regularly and use good quality strings. If you change the gage or tension of the string the truss rod in the neck may need to be adjusted to maintain proper string height.