I think overall weighloss is the only way to do it. Some people just seem to have to shape of face that seems naturally predesposed to a slight bulge under the chin, in which case I'm not sure what can be done.
When I was 16-22 I weighed about 7.5 stone, then when I got to the age of about 27 I weighed 13 stone. I now weigh about 11.5 stone. I have had a double chin from the age of 16 and never lost it.