I figure to get in you'd probably have to be dead, and you'd have to be voted in by a 2/3 majority of living presidents, or something like that. Washington and Lincoln would probably get in pretty quick ... I'm not sure who would be next though.
Alternatively, someone could just tack up a plaque saying 'PRESIDENTIAL HALL OF FAME' over the entrance to a big, empty hall and leave it at that.
What do you think, sirs?