Link Your Twitter Bio directly to an Opt-In Page
Its common knowledge that a link should be present on your twitter page that allows users to be directed to your website, now its time to take this concept one step further. Adding a prominent link that redirects to an opt-in form is the easiest way to encourage fast and easy email signups. This technique allows users to be added to your email db without having to search for the signup form on your website. Don't forget to track the clicks on this link to measure its effectivness comparitively in regards to ultimate conversions.
Schedule Newsletter focused Tweets
A Tweet that prompts users with a question like "Did you miss our newsletter?" or "Check out this great offer located in our montly newsletter" is a great way to increase readership. Even moreso, once the user clicks on the link located in the Tweet they should be prompted to Signup to recieve your mailings so they dont miss out in the future.
Offer A Deal
Offering a deal to Twitter followers that encourages sign-up and forwarding of your offer to friends is one of the best ways to increase reach and gain new users. Some sort of discount, trail, etc that can only be unlocked through opt-in and furthermore prompts the user to share the deal with friends will help spread the word and ultimetly increase your database size.