Using a slapper - which basically is a re-worked horse hoof file and a dolly of similar shape to the panel on the inside of the panel, Careful slapping brings down the high spots and shrinks the steel slightly at the same time. The way this works is the teeth on the slapper as its striking the surface is pulling the metal in towards its self. Concentrating on rolling the dent in towards its self by slapping on the ridge left around the outside of the dent is a sure way of iroining it out.