Coming up with content seems hard. It's easy to worry that what you've written isn't perfect.
The trick comes down to one word: education.
That's right, the trick to engaging and converting customers is education. Don't focus on discounts or cheap copy, focus on sharing your knowledge.
You will likely find this also makes writing your emails a lot simpler. People will forgive a lot of little errors if you're giving them something useful. Some odd grammar or weird structure will go unnoticed in your first round of emails because they're going to be so jam-packed full of great knowledge.
Two great examples of campaigns that educate and are extremely effective are:
WPEngine's educational email course on why speeding up Wordpress is important and why it's tricky (so you should just sign up to WPEngine!)
Despite being in a totally different business, Net-a-Porter's 'The Edit' magazine is another fine example of educational content. Rather than just sharing picture-after-picture of new products they highlight what is in season and give tips on how to look your best. This is much more effective as a sales tactic and much more enjoyable to write and share.
"Sharing is caring" might be an overused saying - but it's true when it comes to email marketing!