Here is a helpful How-To on a site unrelated to animal rescue that has a clear and easy guide to writing effective Letters to the Editor---and the same tips generally apply to writing an effective animal alert:
I would add that in an alert I like to see the point person's contact info, the original author's PERMISSION TO CROSSPOST, the original author identification, the automated date stamp for the original post and the email address from which it was sent. All info is best when it is verifiable as opposed to rumor or conjecture.
I would also stress that the subject line in an online animal alert is best when it contains city/state, type of animal, and the need. When I receive emails that don't contain these facts in the subject line, I know right away that the alert is going to be more difficult and time-consuming to process than others.