I love this post from Copyblogger: Aristotle's Ancient Guide to Compelling Copy.
Aristotle's views on rhetoric profoundly influenced communicators until not so longer ago. They can shape the work of alert fundraisers today if you want them to. Here are three elements of persuasion, according to Aristotle:
Ethos
Readers need to see that you have good character, that you can be trusted. Do that by telling true stories and being completely clear about what you want donors to do.
Logos
That's Greek for "word," "reason," or "logic." Back up what you say with proof. Avoid hyperbole. Be specific.
Pathos
That's emotion. The whole reason they'll give is if you've moved their hearts. This doesn't mean empty, manipulated emotion (that would violate one or both of the other two principles), but deep, heartfelt emotion.