How does one clean a Bowler Hat?

Add some good uholstery shampoo (1001 is fine) to a bowl of warm water then wet a clean cloth with the dilute shampoo, wring out excess water and wipe the surface of the hat working only in the direction of the nap (the texture of the surface) whilst carefully holding the hat in the steam from a boiling kettle. The steam will loosen/soften any soiling and the cloth will clean the felt or satin and lift away the dirt.