Unless you are a purist about using OEM-brand batteries only, Sears has a "smart" DieHard battery that is just the thing for cars that sit for long periods of time. It keeps enough voltage available for startups, and shuts down only if it detects anything in the car that is draining it enough to prevent startups. Goes for about $150. I am thinking about getting one for my Bug, as I have added so much stuff to it, that it would take eons to figure out what gizmo is sucking the life out of my battery...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car