Get a maintenance charger, and just charge it whenever you can (visiting mom, friend w/ garage, anything).
When you get out of your car, lock immediately to put the car to sleep.
You can save a bit here and there by not using DRLs, radio, the vert top, no more BT talking, etc. If and when you must, only when the car is running.
FWIW, I don't really use DRLs, barely use the radio, don't have integrated BT, don't have a vert, immediately lock my doors, and even roll the car out *unstarted* in neutral to wash it. Yet, my first charge took 24 hours. Which leads me to believe that you will need a lot lot lot lot lot of city driving *on one start* to get it up to charge.
Is it possible to remove the battery to have a shop charge it for you, and go from there? (I honestly don't know if it's cool to just remove it with our cars w/o adversely affecting the OBC and stuff . . . I think I recently read it's no problem on the E46?).