If you resort to rudeness or threats, you'll close the doors for yourself and everyone else who shares your opinion on the issue. But, as with everything, there's a right way, and a wrong way. Stick to the facts. In other words, see Step One, and forget about everything else. Editorializing, speculation, and personal attacks are beside the point, and will mark you as a gadfly rather than an activist.
Gadflies make trouble.
Activists make changes. Be sure that the media can tell which you are. If you advocate via e-mail, do your best to compose your own letter.