Use a pointed comb to remove as much hair as possible. Without a comb, gently hit your bristle-side-down brush against a counter to lift all the hair. Use scissors to remove round brush hair. Maximise hair removal. Teeth-comb dust.
After removing dust and hair, loosen the grime and oils on your hairbrush. Get an old toothbrush. Dip it in soapy water. Move it along the bristles to loosen oils and skin cells.
When soaking, loosening is half the battle. Everything must be removed with a toothbrush or scrub brush. Use a toothbrush to clean between the bristles. Rinse. Repeat until debris is removed.