Segment Lists to Personalize Copy
This may not always be possible or necessary, but splitting your email list into segments can help you increase the number of opens and even click-throughs. Segmenting based on interests, focuses, or industries, for example, can allow you to personalize the message of one email to better suit that audience. People are much more likely to open an email they feel is relevant to them.
Remind Them Why They Should Be Reading
If you're following the rules, every name on your email list signed up to be there, though if not they at least have a relationship with you or your business. Make reference to why this list is getting an email from you (another place segmenting can help with tailored copy) to help establish relevance. In today's email age, spam is a big deal - if you can remind someone that they signed up for your list and why, you're much less likely to get marked as spam an unsubscribed from.
Keep the Focus on Them
People are more likely to read through marketing copy they feel speaks to them... so that's exactly what you should do. Use 'you' and 'your' more than 'we,' 'us,' or 'I' to keep the focus on your audience and not yourself. It's similar to most other marketing language; there difference is small, but "we are offering 50% off all weekend" is less personal than "you can save 50% all weekend!"
Benefits > Features
While you should try to keep the copy on your site concise, you have a lot more room to breathe and share information. Email doesn't provide you with all that space; you need to give a lot of thought to what to focus on. Are you launching a new product or service, or just spotlighting your existing ones? Instead of focusing on the features of those products or services, focus on the benefits that your customers could gain from using them. I could tell you that a jug of my orange juice contains one gallon of the stuff and is made fresh from oranges, or I could tell you that drinking my O.J. is a great source of Vitamin C which helps protect your body from the harmful effects of free radicals. One seems a bit more interesting, doesn't it?
As Always, Keep it Simple
Like I mentioned before: email doesn't provide you with as much space as your website, so you need to keep your email content simple and link back to your site. Too much content can be overwhelming and increase the bounce rate of your emails. Oftentimes, people will scan their emails first and mark the ones that look promising for a closer read later. Keep your content concise and your headlines clear, visible, and enticing to allow for easier scanning of your message and hopefully bring more people back later.
Calls to Action
If you have buttons in your email, they're a huge draw for the eye when people are scanning your email. Making sure calls to action describe the desired action at a glance ("Buy This Deal!" "Sign Up Today!") plays into a reader's nature of scanning and can help increase your click-throughs. The same is true for plain text emails: place your links well (make them visible and don't be afraid to repeat) and use compelling copy in the link itself to drive home the desired action since you have no imagery to help you do it.