Dry weather or environments can cause irreparable damage to acoustic guitars, in the form of fine cracks in the wood.
One way to supply additional humidity to your guitar is through a device known as a guitar humidifier. These devices are designed to release humidity inside the guitar through the sound-hole. Sound-hole humidifiers all utilize some type of water reservoir with an absorbent medium such as a sponge, water absorbent floral foam or absorbent polymer. The medium is then surrounded by some type of cover which holds the water absorbent medium in place and restricts the flow of water evaporation from the medium. To get inside your guitar, they slip between your guitar strings and are held in place there. Depending upon the brand of humidifier you purchase, it may or may not touch the wood inside your guitar. Dampit is designed to lie inside your guitar, while Planet Waves and Oasis are completely suspended from the strings.
Water holding capacity and the restrictiveness of the outer cover are the greatest factors in determining how often you will need to refill the device. Needless to say, you cant stick a quart container of water inside your guitar, but...