My car is new so this may not benefit you much but I use the Porsche Wheel Cleaner (which I think is P21S) with success. Just spray the wheels and leave them for few minutes, then rinse. P21S also makes a gel for more stubborn dirt. The gel clings longer and it's more concentrated. Waxing the wheels after cleaning significantly helps against dirt and break dust accumulation.
I still haven't used a brush -- the standard 18" S wheels allow me to use a sponge on the inside with my hands. I apply the wax with my hands, as well (on the inside of the barrel too) -- which ends up being the ultimate proof that my fiance needed to conclude that I like the car's legs more than hers.
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