First, consider what McCain should not look for. Among those irrelevant considerations is a high profile. Nobody, absolutely nobody, is less in need of a running mate with a huge name or a big personality than John McCain is. McCain is so well known, and his "brand" so familiar, and his personality so seared into the public consciousness already that if he chooses a political rock star he will just overload the public's senses.
McCain should not move left with his choice. Some in the media see certain secretaries of state or Democratic senators or "moderate" GOP governors as ideal choices -- but they are wrong. What McCain needs, in order to free himself up to thrust and parry with the Democratic nominee, is somebody who can spend all his (or her) campaign time tending to restless conservatives.