First and For most: Always keep your guitar in a case when not being played. This small step will keep natural damaging elements out of the guitar for long periods of time.

Never keep your guitar in extreme high or low temperatures such as the trunk of a car in the summer sun or winters snow, next to the wood stove or a window which receives direct sunlight.

Over drying of an instrument can lead to sever damage such as fret buzz, cracks in the soundboard, finish imperfections, loose braces and bridges, uncomfortable fret ends on the edge of the fingerboard,etc. There are many easy to use humidification systems on the market today. I advise picking one of them up if you live in an area where the weather is dry or changes through out the year.

To much moisture in an instrument is much better than not; though this can still lead to some issues such as high action and a damping of the guitars tone. Wood is very much like a sponge. It swells when it's wet and shrinks when it's dry.

Saddle: if you live in an area such as new england where you have dramatic changes to the climate through out the year, it may be a good idea to have a summer and winter saddle for your instrument. A lower saddle for the summer months and a taller for winter. This is an easy way to keep playability at its best all year long.

Fingerboard/Bridge: it is also a good idea to oil your fingerboard and bridge a few times a year. I use a natural lemon oil and apply it using a few drop on a clean cloth. Remember it is important to use a natural lemon oil. Other cleaning agents such as "pledge" have harmful chemicals that can dry out the wood.

There are many other situations that may arise throughout the life of your instrument. My advise would be to take your instrument to a qualified luthier at least twice a year for a check up. It's a small price to pay for your investment and i assure you'll be happy you did. So will I.