Send out regular emails to your list. I am on lots of email lists. I get one e-newsletter that can run as long as 5-10 printed pages! I love 'em. I get another newsletter "post card" that is only 200-300 words long. I love 'em, too. It's not the length; but the "constancy of purpose". Keep in contact with your list. (3) Encourage your readers to forward your emails to their friends and business associates.
If you write useful and interesting stuff email list, this is a free and easy way to build a list. People will naturally want to share your newsletter with their friends. But at least a 2-3 times a year, ask for help from readers when doing a "membership drive". (4) Make changing email addresses easy for reader. Place your change of address link in every email. The harder you make people work to change an email address, the more likely they are to just "delete" your message when it comes to the wrong inbox.
Plus if someone is changing jobs and addresses and wants to take your newsletter along with them, let them! (5) When people sign up for any of your services or buy your products, tell them they will receive your emails, too. Most people are more than willing to receive "Upcoming Events", "Product Updates" or "Customer Service" messages from someone who they have done business with in the past.