Word of warning for our fellow classical/ acoustic guitar players: don't forget that during the winter season the air is a lot more dry inside our homes, and so the lack of humidity can be dangerous to our instruments.
I never really bothered about the subject until now that I've bought a nicer classical guitar. While on solid body guitars the lack of humidity isn't that hazardous, translating usually only in the frets sticking out of the fretboard (or, in fact, in the fretboard wood shrinking), classical and acoustic guitars can get shrunken tops and, in the worst case scenario, cracks in the wood.
You can buy a guitar humidifier or make your own using stuff you probably already have around the house, namely a zip lock bag and a sponge. Simply cut some small holes in the bag, wet the sponge, squeeze all the water you can out of it so that it doesn't drip, put it in the bag and place your humidifier under the neck inside the case. If your guitar show signs of dry woods, you can place it in between the strings, inside the soundhole, but beware it can produce a lot of humidity in the air so always inspect your guitar daily.