welcome to my world lol had more bat probs in my friends van than anyone i know.
try a new battery,
get both your current battery and alt tested at discount,
Make sure all the connections are tight and are infact connected.
make sure there's nothing being left on that can drain your battery.
i had an amp once that had a switch (i didn't know about at the time) but you could leave it on all the time, or just when the car was on. needless to say it was on "all time" and my bat kept dieing.

so make sure nothing's draining power/is on even when car is off.