Make that one line that everyone reads so compelling that the reader has to continue to the body of the alert. This is sometimes the most difficult part of the alert, as you try to condense your entire issue into a few words. The headline should not be more than one line long. Put a date on it. Always include the time-frame in which your action alert is valid.
Action alerts can travel through the net forever.
Don't count on the message header to convey the date, as people who forward net messages frequently strip off the header. Make your alert self-contained. Don't assume your readers will have any context for your issue. DO assume that your alert will be read by people who have never heard of your or your cause. Provide background or historical information about the subject of your alert, but keep it short and concise.