This helps establish authenticity of your alert, making is more credible to the activist community, and allows readers to know where to go for more information or clarification. Evaluate your action alert's success. Ask to be copied on emails, letters, etc. When the campaign is over, try to derive some lessons for others to use. Take some time while the experience is still fresh to measure the value of your efforts.
Email is just a part of a larger organizing strategy.
If you want to build a lasting political movement, at some point you'll have to gather people to work together on an issue. Email is simply a tool, albeit a very effective one, in an activist's toolbox. Put clear beginning and ending markers on your alert, such as a row of dashes at the top and bottom, so that any additions made by people other than yourself will clearly be modifications to your original. Use short paragraphs. Email will frequently be read in a document window with scrollbars, making it harder to track long paragraphs.