The power of this approach (to use our consumers' rights in order to make a difference) relies on human communications. Spread the awareness of overfishing!
Talk to your friends. What can be better than sharing a good fish among friends?
Talk to your fish provider, bring him/her information about sustainable fisheries (e.g. eco-certified).
The same with cooks and restaurant-owners. The Seafood Choices Alliance has a lot of information on providers and actions: http://www.seafoodchoices.com/.
Do you write emails? Include a statement about the problem, and a link to a source of information (e.g. this pages: http://evolution.unibas.ch/people/dita/fish/fish.html) in your automatic signature or vist card.
Do you have webspace? Make a webpage about fish! And / or add a link to these or similar pages.
Are you an artist? Take advantage of the beauty of fishes... And let the fishes take advantage of your success.