How long are you driving it for before switching off the engine? I have this problem with my A6, I think because I only do short journeys during the week. The AA told me it takes 20 minutes of driving to give the battery enough charge just to start the engine. My journey to work is 15 minutes, but even if I drive it for an hour or more at weekends and then park it up for the night, just keeping the immobiliser going drains it again. My jump starter is worth its weight in gold, but it is irritating not being able to start the car without it. All the electronics are nice to have but the battery is the size of a small suitcase and the car is craving a motorway run every day!
I considered changing to a 2.0, but if you're having the same problems that's not going to be the answer. If you have found the problem let me know.