It sounds remarkably simple. Who doesn't know at least one trustworthy person who's unhappily voting for "the other" side? If you'd grudgingly vote for McCain and they'd grudgingly vote for Obama, pair up and vote third party instead. Sure, there will be some people who say they're going to pair up and then secretly vote for McBama instead anyway, but unless one side's voters are generally less trustworthy than the other side's, these pact-breaker votes would, in aggregate, cancel each other out.
I also like the "you're not voting for the lesser of two evils, but the evil of two lessers."