I'm pretty sure BMW sell a trickle charger for this purpose.
There is a switch in the boot above the power module and fuse box, you can switch this over if you are not using it for long periods, this may help you. I understand it puts the car into a low power state to conserve battery power if you go on holiday. Im not sure if it will be that beneficial for you if you use it say twice a week. Give it a try.