Choose a winning show. No network will ever resurrect a show that has never gotten decent ratings. The show must have a loyal following and a reasonable chance of survival. It is helpful if there were extenuating circumstances that led to its demise such as boggled time slot, competition with the highest ranking show (preferably with a different demographic) or an exceptionally long hiatus that resulted in lost viewers.
Gear up before the cancellation. Watch the ratings before cancellation month comes around. If a show is on the bubble, it is easier to convince the network of its viewer loyalty preemptively than trying to convince a network to reverse its decision after the involved parties have decided on its conclusion, the cast has been informed and the crew has been let go.
Create a rally point for rabid viewers. A central website is a great place to pass along information, gather opinions and influence viewers to act. Create the page before the show is canceled in your preemptive strike and then communicate a detailed plan after the cancellation announcement has been made.
Rally the town folk. Visit popular television posting boards, announcing the campaign and the actions you are calling for. Encourage people to write to the networks. Contact advertisers with letters supporting the show. Gather ideas on how to proceed next. Find out what people are doing already to save the show.
Flood key decision makers with letters of support. Post the addresses, phone numbers and emails on your website. Include a sample letter so people know what to say. Encourage everyone to be polite and respectful in their tone. Suggest they include their name, location and age to help the network expand their view of the demographics.
Concentrate your efforts. Don't piece meal several different strategies but throw your weight in one direction to make the biggest impact. For example, in "Jericho's" campaign, the fans mailed nuts to network officials because of a tie in with the last show. They partnered with an online distributor, posted YouTube videos of the delivery and used "Nuts" in every ad and press release. It fit with the show and everyone could contribute so it made a concentrated impact.
Write press releases announcing your fight to keep your show alive. Focus on the exceptional things you are doing to bring back the show. Distribute the press releases through traditional avenues as well as online ones. Include contact information for further questions.