steam

Using a heating element is risky. You are better off using one of those ultrasonic fogging inserts they use in many indoor water displays. Look in craft shops (on the net) to get just the insert.

http://www.google.co.uk/products?q=ultrasonic+fogger&oe=utf-8&rls=org.mozilla:en-GB:official&client=firefox-a&um=1&ie=UTF-8&ei=AcEtTM2uEILw0gS5k72eAw&sa=X&oi=product_result_group&ct=title&resnum=1&ved=0CDIQrQQwAA

or
http://www.amazon.co.uk/Maker-Pond-Fogger-Humidifier-Vivarium/dp/B002YONWVO/ref=sr_1_10?ie=UTF8&s=garden&qid=1278067008&sr=8-10