To get a BCS berth they have to either:
- Win C-USA,
- Finish in the top 12 of the BCS rankings, and
- Finish ranked higher in the BCS than the MAC, Mountain West, Sun Belt, and WAC champions
or
- Win C-USA,
- Finish in the top 16 of the BCS rankings,
- Finish ranked higher in the BCS than any one of the ACC, Big East, Big 10, Big 12, Pac-12, or SEC champions, and
- Finish ranked higher in the BCS than the MAC, Mountain West, Sun Belt, and WAC champions
So basically win out and they're in, unless Boise State manages to win the Mountain West and stay ranked above them.
It's also theoretically possible that they could get an at-large berth if they don't win C-USA, but extremely unlikely since they would drop in the rankings with a loss.