I had a similar problem with my car. My battery died and I took it to a local auto parts store that will check out your electrical system for free (O'Riellys, Auto Zone, NAPA, etc. ). They said on this year/make/model that the Alternator has an electric "valve" that opens to restore a charge to the battery. When you shut your car off, the "Valve" was sticking in the open position and the alternator was actually sucking life from the battery. I had to install a new alternator, and a new battery because it had killed a couple of the cells. So my advice is take it to your local parts store and have them diagnose it, then follow their advice.
Pontiac Is Life, December 2007