Sometimes it's easy to get carried away with a great idea or campaign and forget about the basic rules of marketing communications - this is a good reminder of how to get it right from the start.
Commandment #1: Know your audience. Who is this e-mail going to? Picture the average person on your list. Give them a name, even. Think about what their day is like. Think about what's important to them. What are they passionate about? How old are they? What products or services have they purchased from you in the past and why? The more you know about the audience you're writing for, the more targeted and relevant your copy will be.