Easy. The teams pick. Win your conference championship game, you get in. As far as the 4 at-large picks that should be rather easy as well. Take last year for example. Florida gets in as the SEC Champ. Alabama should also be included because of how highly they were ranked & because of their record. Oregon St. was the Pac-10 Champ if I'm not mistaken, so they're in. USC would have been included for the same reasons as 'Bama. Oklahoma got in because they were the Big XII Champs & Texas gets in for....the same reason as 'Bama & USC.
Essentially the 4 at-large teams will be the second best teams in major conferences as smaller conferences usually just have one team good enough to be in the playoff & they will be unless they slip up & lose their conference championship game.
In other words, you want a team from your conference to represent you in the playoff? Join the program & have a conference championship game.
That makes the regular season important because you want to get into you CCG....you want to win your CCG because it automatically gets you into their playoff at a higher seed....sooooo....there is ZERO reason to not have a playoff....none.