My rules are pretty simple: with a few exceptions (I don't care if the Royals won the World Series last year, I'll want them to win this year too), it's "Who's gone the longest?" I can get pretty convoluted, too. For example, even though the Astros have never won the World Series, if it came down to Houston and the Pirates fighting it out for the final National League playoff spot... I'd be pulling for Pittsburgh. I know, I'm fuckin' weird.
Anyhow, here's my lists, updated after each season wraps up: