The adjacent picture (taken from our accredited auto-detailing location JREK Autocare) illustrates before and after utilizing a clay bar during the wash process to remove brake dust.
Clay Bar (as seen from the below picture) is a bar of clay that you rub by section on the paint of your vehicle during the wash process.
From optimum results, you'll want to fold and/or twist the clay bar after you've rubbed a particular section of paint. This enables the clay to replenish to a newer portion of the bar.
Typically, a bar lasts about 6-12 vehicles (depends on how dirty paint is and how much time you spend claying a vehicle).
Of note, the longer brake dust has sat on paint the harder it is to remove. This means you may have to clay a vehicle multiple times before achieving your desired results.