Usually your list members will have friends that have similar interests. So, why not encourage your readers to send the link to your opt-in form to their friends? You might give them some kind of incentive to do so, as this can dramatically increase the response.
This is another example of social proof at work. Because the recipient is getting a recommendation from their friend, you'll be a lot more likely to get the sign up.
You can simply put some text in your messages that encourages them to pass your newsletter or opt-in address along. There are even website scripts out there, such as the “tell a friend” script that will allow them to easily enter their friend's e-mail address on the spot.