I bought some gadgets that are designed specifically for this purpose a while ago after my DCX-1E developed a crack right down the middle of the sound board. They're these little sacs made out of a material that looks like felt and they have a cap on them which one unscrews. Then you fill the sac up with water cap it, and leave the whole thing in the case with the guitar. Water evidently slowly seeps through the sac at a rate that keeps the environment inside the case sufficiently moist. It really does seem to work and I do find that they have to be refilled regularly during winter time.
Actually, here's a link to the gadget I use - at least I think this is it:
http://accessories.musiciansfriend.c...ier?sku=421665
-DM