Is it a new battery?
Or is it a known problem with your car and other batteries that it has had?
A friend of mine had his new battery go flat, just as by sitting there for a few days, apparently there was a short inside the battery and it was flattening itself.
Otherwise..
have you got a Multimeter?
Check the current that is being drawn with the ignition key off, maybe something is really hogging power when you park it up..