I always recommend that authors have a "fan club".. usually a mailing list of dedicated followers. Escpecailly if you can get them to take action (like buying your book on a certain day to promote it to a higher level on a best seller list)
Potter books don't need this sort of coodinated activity.. but the books do inspire fan clubs. That's a boost even for the number 1 book..
How coud this work for you?
Lumos 2006 - Home: "Grab your broom, set your Portkey, or prepare to Apparate! From July 27-30, 2006, Las Vegas, Nevada is the place to be for Harry Potter fans near and far as we gather to attend Lumos 2006: A Harry Potter Symposium. Following our theme of 'Back to School,' we are planning a variety of daytime academic presentations and panels, including special workshops for educators and librarians. In the evenings, we'll be having fun with an assortment of activities, including Water Quidditch, Hogwarts Night Classes, and Live Chess.
Prepare to be completely immersed in the world of Harry Potter. Our host hotel will be decorated Hogwarts-style, with a 'Great Hall,' a 'Common Room' for all houses, and a library for those wishing to emulate our favorite bushy-haired witch. We'll also enjoy British food and butterbeer throughout the symposium.
This event will mark the third such offering to be sponsored by HP Education Fanon, a non-profit educational organization. "