The most effective way to build and maintain an email list is to offer educational information about your industry. If you are an attorney write short newsletters about identity theft, law enforcement confrontations, business incorporation, etc. Regardless of what industry you might be in you can always find good relevant content to send to your email contacts.
In addition to the educational information you also need to take advantage of your email campaigns to promote events, promotions, and products or services. Use the sidebar of your email to talk about events or sales that are upcoming. Promote your Facebook or Twitter profiles and encourage readers to share the email with their social networks. You can also attach coupons or specials in the email. Banner ads for your affiliates or business partners can also be effective.