BookEnds agent Jessica Faust (whom I have worked with as both an editor and an agent through Alpha Books) recently posted her advice to recent graduates hoping to get a job in book publishing (read it here). Basically Jessica says you need to move to New York City, where the big houses are, and also consider applying for jobs with literary agents. You know how I bristle at the first point, but fair enough. Your odds probably are better there--although the downturn and the digital revolution are turning all that on its head.
Commenters added another good point that I have mentioned before: Work in a bookstore. We're suckers for people who can profess to bring us knowledge of how booksellers think.