Batteries are consumable items... meaning - they do wear out. Eventually, your battery will not be able to retain a charge. Replace it and move on!
What I would do is open the trunk and disconnect the positive lead. Charge the battery overnight, then use an ammeter between the positive contact on the battery and the positive lead. See if any current is being drawn. A couple milliamps would be OK, as the alarm system and radio draw a little power. If it's something significant, it could be that something is not turning off like it should.
If you get another battery, go with the Optima Yellow Top. I don't know what size, but check around for some online vendors. They are a little smaller than the OEM battery, but usually come with a small plastic adapter. Some local retailers toss the adapters, so be careful if you get one. Do a search here for 'Optima' and you'll probably find a few threads discussing these batteries. They are recommended because they have a very good capacity, and you could operate your top with the engine off, and not worry about draining your battery too badly.
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2001 Boxster - Grey on Grey
1969 911T Targa - 'Stinky'
http://www.zoto.com/frayadjacent/img...f27a-4a399.jpg <---- my car. ^ crap I post.
"The existence of the flamethrower is evidence that someone, somewhere once said 'I want to set those people over there on fire, but I don't want to have to walk over there to do it.'"