Storing a vehicle - battery issues

When storing a vehicle, or particularly keeping an extra vehicle for emergencies, is there anything special that should be done? The reason I ask is that I have had two vehicles' batteries die while I was storing them.

The first was brought over from Hawaii and had this anti-corrosion thing under the hood that was hooked to the battery and may have been draining the battery. I replaced that battery and it was dead again within a month or two. I got a new battery, kept it disconnected and it was fine later.

The second, my wife's old vehicle, has been in our garage for about a year and I had last taken it out about 6 months ago. Battery was dead when I tried to start it last month, but I noticed that I had kept a GPS unit plugged in and thought maybe that drained the battery. I jumped it and ran it a bit but had to jump it again today. Measured at 7 volts (should be 12) at the shop, had to get a new battery. Crazy thing is, they (Sam's club) couldn't install the new battery for me and couldn't jump it, neither could AAA. I got it towed home and changed the battery out myself. Incidentally, I tried jumping it myself again and was able to using one of those new small portable batteries that can jump vehicles.

So should I just keep the battery disconnected? Should I just try to take the vehicle out every few months? Also, it's an old Mercedes and the car is bugging me about us being overdue for service by like 230 days. Do you guys still recommend taking it in for routine service if I'm just taking it out for a few miles every several months? Thanks for any replies.