[patrickt]

Pull the negative cable off of the battery and attach an ammeter between the negative cable and the negative battery post. With everything turned off, if the ammeter is reading over 25 milliamps, something is sucking the juice out of your battery. Start pulling your fuses, one at a time, starting with the lowest amp fuses first. The fuse that stops the current drain is the circuit that is causing the problem. You then check each device on that circuit. On non-Cobra cars, common parasitic offenders are the little light bulb switches in the glove box, trunk, etc. The ammeter will tell you what's doing it though. My car battery sits from Thanksgiving to the Spring without any maintenance, charging, etc. and the car still cranks hard and then fires right up.