Make it personal. Pretend you're sitting down to talk to a friend. Let your message be friendly, animated, and informal. Your English teacher will never read it, and he wasn't teaching you to write advertising, anyway. Rather than thinking of yourself as selling a product or a service, think of yourself as providing benefits .
Benefits are different from features.
Be clear in your mind about what you want the reader to do after reading your message. Let the user know what response you want, and let them know in a compelling, urgent, benefits-oriented way. Include complete contact information, and triple-check it for accuracy. I suggest that you offer multiple ways to respond. For example, many people respond to the mailbot for my Internet marketing book by e-mail.