Its one of two things: The battery, or the wiring from the alternator to battery is bad.
Before you take it to get looked at just make sure you take a wire brush to all the terminals and stuff - disassemble the connector and clean the ends of the wires too if you can. Paid for a tow one time to have my mechanic clean everything up and it was good to go. Still not living that one down...
I remember one time replacing the battery and alternator in a buddies car, only to realize later that day when it died on us at 7-11, got jumped, then died again once we got back, that the wiring was exposed and grounding out. A few pieces of tape later, it was all fixed....
Although chances of that actually being the problem are like a million to one... So its probably your battery. lol