To retain you sign-ups once they have opted in you need to continue to provide useful and quality information. Do not just email your prospects offers without mailing list offering value, mix up the content you send them, sometimes just send information without any call to action. Build their trust and then subtly offer products to purchase.
Treat them with respect and see them as you auto-pilot income, your own little gold mine. Not to be depleted immediately but a long term mining operation, carefully mining each field and not destroying other fields on your way. Remember that your opt-in list is mailing list the foundation, your solid base of a potential profitable on-line business - your auto-pilot income, which is your ultimate goal.
Every Internet marketer struggled with the same problem when they started their business. How to get traffic to my own website? Nowadays a lot of so-called "gurus" try to sell you their products as the number 1 solution in your search for targeted traffic. But when mailing list analyzing carefully their techniques and tactics, you'll discover that they too act always in the same manner: they are building their list.