Guitars are sensitive to their environment. They react greatly to fluctuations in humidity and temperature, and they need to avoid being wet. The wood should not be scrubbed or rubbed with a dripping sponge or the like. It is also imperative that your fretboard be cleaned regularly as well, as it can pick up natural oils and grime from your hands, making the instrument look and sound suboptimal.
If your guitar has a lacquer finish, feel free to rub it with a damp cloth and perhaps some dish soap. If the guitar is not lacquered, the damp rag should be avoided, as the wood can absorb the moisture and swell. The strings and fretboard require a little extra care. The best way to clean sweat and skin residue from the fretboard is to relax the strings and then remove the dirt with a fretboard cleaner such as the Fretboard Cleaner from Ernie Ball. Putting a little fretboard oil on a cotton cloth every now and then and rubbing it on the fretboard after playing is helpful in the long run. The strings can be cleaned with a product such as String Cleaner from Ernie Ball. Another large factor in the condition of your guitar is the storage of the instrument. Cleaning your guitar regularly and keeping it in a proper case will go a long way toward keeping it in good shape.