Two ways you can keep me as a subscriber:
1. Remind me what the hell it is your company does/sells in the first sentence. Between my signup and now, chances are about 95% that I forgot what I even signed up for.
2. Make it easy to unsubscribe. That means a very visible one-click link. Unsubscribing doesn't mean I'm no longer interested in your product. But making it difficult to unsubscribe says a lot about how I will be treated as your customer.
Go easy on the HTML / CSS / Images.