These people on your list will develop a relationship with you and you could develop their trust, they will be more likely to purchase your products mailing list as they become available, this list becomes your lifeline. Over time you could build a list of any where from 10,000 - 50,000 people, as long as you always offer strong relevant content and other promotional items for free.
People on line are there because they are looking for information, after you get people to Opt-in to your mailing list, to keep these people you have to send them current quality information that they can use. If they like the content they may put it mailing list in their blog making it accessible to hundreds of other people, remember to always have a backlink to your site.
To expand your mailing list your Opt-in button / form must be in a strategic place on your page, usually in the top left quarter and it must also be on every page of your site, so visitors do not have to leave the page that they are on. Opt-in forms should mailing list not ask for too much information simply a Name and email address is enough. To increase registrations, and Expand Your Mailing list.