If you perform a search, you are trying to get data from the server, rather than sending data to it - this means that you should probably use GET, not POST.
In addition, it will allow users to bookmark results pages (or send these links by email, IM, ...), which is always nice; as well as use the "Back / Forward" buttons of the browser without getting a warning window, which is also good. ^^
(, , , ;-))