Make the simplest humidifier by filling a pot of hot water and placing it on top of your radiator. Apartments with old, hot radiators tend to dry out the room. If you do not have a radiator, skip to step 2.
Fold over a Ziploc bag several times and punch several rows of holes. Do not worry if you aren't able to punch all the way though, even a partial punch will allow the homemade humidifier to work.
Wet your sponge in the sink and ring it out until all the excess water has been squeezed out. Place the sponge in the bag and seal the bag up.
Put the bag and sponge in a dry room in your house. Repeat steps 2 through 4 with additional sponges and Ziploc bags for each of the rooms in your house. You should notice a difference within a day.
Repeat steps 3 and 4 each day to renew your sponge's moisture and to continue to humidify your house.