Clearly identify the boundaries of your alert. Use high-visibility lines of asterisks or dashes to show where your alert begins and ends. Keep it intact. Include a clear disclaimer that politely requests the alert be left intact if forwarded. Clearly identify yourself, your group, your sponsors, your mission.
Writers often presume that their return address infers clear authority.
Do so in your alert. Clarify who among your audience will need this information. Date your alert. E-mail chain letters and petitions have survived for years after their topics have expired. They're preserved almost indefinitely in cyberspace, like virtual Twinkies.