My '04 325xi is only about 3 weeks old, so I'm still anal about the brake dust.
I've been taking a bucket of water and a microfiber towel every couple of days, wiping off the brake dust with the wet towel, then doing a quick dry with another towel.
Pretty painless and quick, makes 'em look like new again.
At first I was using paper towels with no water - but the water and a towel makes it come off far easier.
I hope I can manage to keep caring this much about it for a long, long time. It really is painless, and it looks so much nicer than those brake-dust-covered wheels you see so often. It is astounding how much dust these things generate.