According to Political Science 101, running mates are supposed to contrast the the main candidate. And if the current political parties followed that advice (by selecting perhaps a female or minority candidate), they'd greatly boost their chances of winning too.
But, in the last several elections, it's pretty much been old white (usually Southern) guys being paired up with other old white guys with similar political philosophies (Bush-Quayle, Reagan-Bush, Clinton-Gore, Dole-Kemp, the list goes on...) Polls showed that had Dole run with Colin Powell, he would have probably won against Bill Clinton, but he made an aweful choice for his running mate.
I wouldn't expect anything different this year. The current Presidential election sucks, neither man seems like the American public's first choice for President. They've only gotten there because they're "insiders" (John McCain was clearly more popular than Bush with the American public, but not so within the top Republican circles). I'd expect their running mates to be insiders of some sort too. When the final announcements are made, they're running mates will probably be guys you've never heard of.