For most of us in the U.S., this time of year means the air is starting to really dry out. Last November we talked about the importance of humidity to your guitar. Sure, you can keep the humidity up at 45% in your house...but in the cold weather you'd have water running down the inside of your windows, so scratch that. You can also dedicate an entire humidity-controlled room to your instruments. Or...
There are plenty of products you can purchase to humidify your guitar, and you don't have to spend a lot of money. We personally like the Planet Waves GH Acoustic Guitar Humidifier; it's easy to use and works well...and it only costs 7-8 bucks. That's not your only choice though. Planet Waves also makes a two-way humidification system that adds or removes moisture as needed to keep your guitar at 45-50% humidity for about $20. We haven't tried this yet, but it sounds pretty cool.
The Oasis OH-1 Guitar Humidifier uses a polymer gel, which they say reduces the risk of leakage of moisture into your instrument (about $20), the Dampit Acoustic Guitar Humidifier (also about $20) is a tube-style humidifier with a humidity indicator, and if you're really looking for humidification on the cheap you can always use something like the Herco Guardfather HE360 Humidifier (about $4), which uses a clay-like substance that holds moisture and slowly releases it.
Generally speaking, spending more on the humidifier will mean less maintenance of it on your part, and tighter humidity control, but ANYTHING IS BETTER THAN NOTHING.