Make it shareable. Send content that people want to share, and make it easy for them to do it. Sure, subscribers can forward your campaign to friends, but that's a lot to ask. Include a public link to the web version of your campaign so people can read it outside of their email programs, and consider adding Twitter and Facebook links to your newsletter, so readers can share your content where they're already active. When their friends start sharing and subscribing, you'll know it's working.
This article is by far one of the most interesting and fun articles that I have read in a long time. I say fun because the points mentioned here are simple and easy to read. They are not long passages that speak a lot and tends to lose focus. On the contrary the points mentioned here were in short crisp sentences that did not bore me but made my read a pleasurable one. Thanks!