Have the champions of the three highest conferences (plus one wildcard), but make it manditory that all three must fall within the first six (or eight, ten...whatever they choose) places in the BCS standings to be considered, otherwise the highest ranked teams that didn't win their conference.

No conference can have more than two teams. (Don't want a rigged play-off with four SEC teams - don't laugh, I'm sure ESPN would love to skew the polls to do it.)

Also if one conference places a second team into the four team mini-tournament, they must play each other in the semi-final...that way you'll always have teams from different conferences in the finals.