Battery discharging while in storage

[DeltaMustang65]

I know this is probably a simple problem, but I have to ask because I've never stored my car before and I'm wondering if I'm doing something wrong. Since last fall, I've had my mustang in the garage. I changed the insurance to the basic stuff (fire, theft, etc) and I haven't driven it since last fall. I start it up weekly for about 10 minutes just to keep things nice and oily. My problem is that about a month ago, the car didn't quite want to start because the battery was weak, so I ended up jumpstarting it. I also unhooked the cables from the battery after that, thinking that something was sucking up the juice. Then last week, the same thing happened, but this time I ran the car for about 20 minutes instead of 10 in hopes that the alternator would charge the battery up more fully. I guess what I'm asking is how do I make sure I don't kill my battery when I'm not driving the car regularly?