Two suggestions that would help you a lot:
Get a "battery tender" and leave the car hooked up while it's parked. A battery tender constantly measures the battery voltage and only runs a little charge into it if the level starts to drop off a bit. Batteries self-discharge readily and will go flat from just setting, and this is hard on them. My car sets most of the time but the tender keeps it ready to go. My batteries also last for inordinately long times too -- as much as 3X normal lifespans because they're always in top charge condition.
You can also disconnect the battery if the car's just sitting. Almost every car has some minor load somewhere -- the clock, for example -- that pulls it down constantly. While it's a good idea to find excessive loads (like trunk lights that don't shut off, or a glove box light that stays on) you will always have the minor ones to deal with. Disconnecting the battery from the car cuts you off from those loads, although it still doesn't eliminate the "self-discharge" problem described above.
