I discovered a great article on writing persuasive fundraising copy on the KISSmetrics blog.
Their first key is to write conversationally. People buy from (or donate to) people that they like. The way to get readers to like you is to write the way you talk, rather than trying to write a hyped up sales pitch or a more formally structured case for giving.
Conversational writing that projects your personality, sincerity and likability is not easy. The article suggests that you talk naturally to a close friend about why you believe in your nonprofit. Record the conversation. Then transcribe and lightly edit it to create conversational copy that projects your likability, sincerity and beliefs about your nonprofit and its programs.
The second key is to use stories. Most of us are really bad at processing vague ideas, abstract concepts or relationships, and unordered sets of things. But as soon as we have a real world example of something, we find it pretty easy to generalize it to other situations.
The third key is to present benefits and then features in your illustrative stories. The benefits are the reason why someone should donate to your nonprofit. The features of your nonprofit’s program make it believable that your nonprofit can actually deliver those promised benefits.
The fourth key is to use lots of hooks in your copy. Most people will read a little bit and then start scanning. The hooks (subheadings, bullet lists, images, boldface, etc.) tend to stop the eye from scanning over too much of your text.
If you want to write more persuasive fundraising copy, I recommend that you to read the entire article at http://blog.kissmetrics.com/copy-without-gimmicks/