Put damp towels over a hot radiator (Not electric). The moisture in the towels will prevent the air in the room from becoming too dry.

Also you get warm dry towels. Mmm

You can also buy room humidifiers that pass the warm air through the machine and add moisture to the dry air. Try the search I did a Jeeves above, it lists loads of variations to the room humidifier.

There are also little china pots that you can hang from your radiators that you fill with water. They are on the same principle as the wet towels but I am all for the warm wet towels myself.