1. Don't send emails more than once a week. I'm immune to all marketing emails, if they send me a mail more than once a week. I'm sure that I'm not the only one?
2. Would you buy anything yourself from the email your are about to send? Be honest!
3. If you are sending emails before/during holidays. Make sure you have saved your best deal for that.
4. Ad an easy to find unsubscribe button/link