My previous car, a BMW E36 used to be horrible with brake dust until I changed the brake pads to those that were recommended to me on a BMW board. Worked great, didn't have to do much besides minor wiping and some spraying carefully with a hose. But the OEM pads were notoriously dirty. One could easily see who was an enthusiast by looking at their front wheel and seeing how dirty it was.
I wonder how the Subaru pads are...
The BMW guys seem to use something called P21 that I've seen a couple times and I've heard that waxing them will keep them cleaner longer too.
If you wax the front wheels, it has the same effect.
Quick and dirty method....
Use some of the "spray detailer" waxes if you want something quick (only do it while the wheel is OFF THE CAR. Spray on, wipe off.
Long term method....
Use normal paste style car wax. Apply it to the wheels just like you would to the body of the car. (wax on, wax off). The brake dust and dirt just rinses right off.
The spray on stuff doesn't last as long (usually need to re-do it once a month), but the full paste wax method works for between 3-6 months depending on climate (basically, wax your wheels whenever you wax the rest of your car).
Only downside to this is if you ever do track days, where your tires/brakes/wheels get very hot, the wax coating will soften, then the brake dust ends up embeded in the soft wax. Then it's soap/brush time to remove the wax/brake dust built up.