Toyota Repair: 1998 Lexus LS400, lexus ls400, negative cable

Question
I have bought several batteries over the years for my car.  The battery goes dead, sometimes after just a few months.  Can you tell me where to start to see what's draining the batteries?  

Answer
There may an excessive current draw on the battery when it's not driven, the first step is to connect a multimeter and set it to read amps, the battery negative cable is disconnected and the meter leads are connected in series with the battery terminal and the negative battery post. A current drain of more than 500 milliamps(0.05)amps is excessive and the cause has to be found, it could be something as simple as a trunk light staying on or something more complicated. If the current draw is normal then have the alternator output tested.