Getting people to sign up for your email list is as much "selling" as any other part of your business. People "pay" for valuable content by providing their email address.
That means you need valuable content to give them - before they ever receive a newsletter.
(Side note: how often are your newsletters valuable to subscribers? If you aren't providing information they care about and that will build relationship and sales, what are you achieving with your email list?)
The thing you offer in exchange for an email signup is called a "Lead Magnet," and Amy Porterfield has a comprehensive summary of the different kinds of lead magnets you can make, with all the pros and cons of each. Listen to the podcast episode, or read the transcript, for some ideas of how to build your email list.