What would it take for a non BCS Conference School to Play in the BCS Championship?

I know BYU won a National Title like 30 years ago. But in the current BCS; I'm guessing the only way a non BCS conference school like: Utah; Boise St. or Ball State could play for the national title is to have a undefeated season, play in a BCS bowl game, win, come back the next season and remain undefeated (23+ winning streak) with wins over BCS conference teams. Then maybe if all the other BCS schools have at least 1 loss, it could be a possibility.