Enter the FlyLady car duster. I read about it several years ago and have used it ever since. It is a cotton yarn type duster that has something in it to attract the dust. I sweep it around in my car a couple of times a week and it keeps my dashboard looking spotless. This last year my duster disappeared (I found out that it was kidnapped by my husband), so I asked for another one as a Christmas present.
So, now each of our cars has a duster stored under the front seat and clean car dashes in our future.