Here's a great set of principles from the Googledigook blog: 17 things I have learnt about charity email copy.
Here are some of them -- the ones that apply to fundraising writing in general, not just email:
- READ IT OUT LOUD -- if it doesn't sound like a real person speaking, start again.
- Cut, cut and cut again. If the meaning remains the same, you've almost certainly made it more elegant by cutting.
- You are allowed to begin sentences with 'And' or 'But.'
- There should always be some version of the Call-to-Action above the fold.
- Avoid sentences with multiple clauses and sub-clauses -- it's what we learned to do at university, but it's awful copywriting.
- Steer clear of adverbs. They're unnecessary.
- The message must be about the recipient, not the sender. Always talk about 'you,' never 'we.'
- Email content is a less-than-zero sum game. Talk about three different things, and you won't get three times as much engagement. You won't even get the same amount of engagement, split three ways. You'll get less in total.
Good stuff. More at the original post.