Someone's bound to mention it in this thread (or I hope they do, because I thought it was great but can't remember the name or the details) - but there's some formula that's roughly like:

  N = total applicants
  1. Interview N/10, decline no matter what 
  2. Hire the next person who's better than everyone seen so far

As I recall it was slightly more complex than that, perhaps only to give it an impression of being rigorous theory, but that was the gist of it.