The best way to find a love hotel in Tokyo is to take the JR train line (not the subway) to the Shibuya district (get off at the Shibuya stop). Then, just wander around and make your choice. For more information, check out the exhaustive discussion of love hotels in the September edition of Japanzine (http://www.japan-zine.com) or the Quirky Japan Web site (http://www3.tky.3web.ne.jp edjacob).
They forgot to mention the exhaustive discussions on FG.