Another good point that I learned on here a few weeks ago (not sure who said it, but thank you very much) is that you should be unplugging ALL 12v devices - EVEN IF THEY ARE NOT ON.
Case in point, our Dodge 3500 CTD. It is supposed to have two 1000CCA batteries, and I didn't realize that until I checked to see what the replacements were. The PO had two 750CCA batteries in it. The thing would also never start at below -15....plugged in. Well, it warms up, you never get around to replacing them, we hardly use it right now....and then I read about the unplugging 12v accessories post.
Out comes the satellite radio, out comes the GPS, and out comes the remote fans installed in the back for the dogs...none of which have been turned on in the past three months, as it's been parked.
I boosted the dead truck (no surprise there) three weeks ago, and it's been fine since. Starts at -33 no problem too.