Attract the user within a few seconds and keep the content relevant. In some ways this almost goes without saying, but someone is likely to look at your email for only a few seconds before they decide if they need to read it or not. Keep make sure you have something (a great headline, a subject, etc.) that will attract them quickly. Then make sure that your content is very relevant to the user, or else you'll risk turning them off to future emails you send.
Avoid bad words ('Free', '$$$', 'Save', 'Discount'). These are likely to trigger spam filters, and aren't likely to increase your open rate. MailChimp
did a study of open rates for various subject lines, and found another three words to avoid: 'Help', 'Percent off', and 'Reminder'.
Personalize your subject lines. In an informal study I did, I found an 18% increase in open rates when I used 'Hey [name], XYZ' instead of just 'XYZ'.
Aim for character limits on subject, preheader. In general, you want the 'From Name' to be less than 25 characters, the subject to be less than 35 characters, and the pre-header to be less than 85 characters.
Display the recipient's name in the body of the message. This helps let them know that you know who they are, and the message isn't spam (spammers aren't likely to know the recipient's full name)
Make sure your sender information is correct. For the 'from' name, use a company or brand name, and not an email address, generic department or unfamiliar person's name.
Consider all of the following features, as your technology platform allows:
Forward-to-a-friend link: A button or text link to an online form to send a copy of the message to another recipient.
Opportunity to subscribe: Provide a button or link to a subscription form. If your subscriber forwards the email to their friend, it would be great to get the friend to signup too.
Preference Center/subscription management area. Let the subscriber update their email address or change their preferences rather than completely unsubscribe.