Things to consider for maintaining your stringed instrument in good condition.
Your instrument needs proper care to stay in good, stable and playable condition. How you treat it and the environment in which it "lives" have a big impact on its well being. Below are a number of best practices and some underlying information which should help to explain the "whys".
Humidity
Fine instruments are made primarily of wood that is glued or otherwise joined together. Wood is a material that "moves" as a result of changes in it's moisture content. As it gains moisture, it expands and as it looses moisture it shrinks. The moisture content is affected by humidity and temperature. I have a reference table here which shows the woods "equivalent moisture content" as it relates to the relative humidity and temperature of the surrounding air. For example in New England a winter day in a heated house my be 60 degrees and 25% relative humidity. A summer day may be 95 degrees and 75% relative humidity. Looking at the table you will find that the moisure content of the wood will vary from 5.4% to 13.9%. You can look up values for your area. As you can see though the change can be dramatic and this has a damaging affect on your intrument. So it is important to limit this as much as possible.
Things you can do; Maintain a proper environment for your instrument.
Temperature
As you can see temperature has an affect on humidity and moisture content of the wood. There are other reasons as well to be concerned about temperature. Rapid changed of temperature as well as exposure to extreme temperatures can cause problems as well. Many glues are weakened by heat, especially while moisture is present. This can result in joints that "creep" or move as well as joint failure. Having one joint fail can at times transfer stress (such as those from string tension) to other joints setting up a chain reaction of failures. In the worst case the wood itself could be warped or cracked as a result. When subjected to extreme cold, cracks or "crazing" of the finsh can result. Either extreme in temperature is bad and rapid change from one extreme to the other is worse.
Things you can do; Avoid temperature extremes.
Finish Care
The best way to maintain the finish of your guitar is to keep it clean. Wipe off perspiration and fingerprints with a soft, damp (not wet) cloth. Old, cotton baby diapers make very good guitar cleaning cloths. A very lightly dampened rag with plain tap water and thoroughly wrung out will remove most dirt. Then buff with dry clean cloth. Use caution if you use commercial products. Many contain solvents, silicones or abrasives and should not be used. Solvents can damage the finish. Silicones contaminate the finish making repairs very difficult. Abrasives in polishes although very fine, remove finish along with dirt.