Presidential candidates should pick vice presidents who help them with the actual governing part of being a president, since they don't help with winning swing states.
John McCain should pick someone who makes up for his egregious inadequacies as a potential president, the fact that he's beholden to federal lobbyists, his virtually uncontrollable temper which makes him vulnerable to being manipulated in politics, and his relative ignorance and knee-jerk belligerence in matters of foreign policy. To help him with all these very, very many weaknesses, he should pick someone who's younger than him and is known as very virile and healthy man, and someone who knows a lot about being easily manipulated and about the dangers of a warmongering approach to foreign affairs. Whether they're from a swing state does not matter in my criteria, so he should choose George W. Bush.
Barack Obama, meanwhile, should find a VP who can complement so impressive a candidate without too much overlap. He should find someone with a solid military background, someone with government experience but not a lightning rod or a reputation for being highly partisan, preferably a former Republican. The candidate should also be tough and "no nonsense," who conveys the impression that this administration has the big stick (!) that helps so much in political matters. Swing state affiliation doesn't matter, again, so the best candidate for Barack is Jim Webb.