could be a couple of things:

1. alternator is going and not charging the battery properly
2. battery has gone to shit
3. theres a short or something drawing on the battery even with the vehicle off

first thing i say is take it to a parts store or service shop and ask them to test the battery. a good store should be able to test for free on the vehicle, also many of them can perform a basic alternator test on the vehicle. any quality shop will do both with a quality charger for a nominal fee.

if both pass the tests ... most likely something drawing juice.