I use bilbery cleaner diluted to the weaker end of the recommended amount, shifts most brake dust without fuss and is much nicer to work with than wonder (where all my laquer went) wheels.
Would be nice to change the pads to something better performing that BMW oem and less dusty, the oem pads create a ton of dust.
I'm with Popey on this. Couple of good coats of Poorboys from time to time and then just normal car shampoo with regular washes and have no problems with brake dust.
Or switch to low dust pads and the problem of cleaning BMW brake dust from your wheels goes away. I have Akebono Euro pads and I can't believe how much cleaner my wheels are.