Offline, ask each person who walks through your door or phones for information if they'd prefer to be on your physical mailing list or your e-mail list, and be sure to mention that you sometimes offer deals or discounts only to those on your prospect mailing list lists. That's how to build your mailing lists, so now let's look at building a bigger customer base using them. Since you know those on your mailing lists are targeted prospects interested in your particular niche, it's easy to judge what information, products and services would be of interest to them.
Use your mailing lists not just for hard-sell pitches, but instead to educate your prospects about the various topics, introduce them to new products or services you're offering, and give them special time-limited offers available only to those on your lists. That combination of information will do numerous things to help with building a bigger customer mailing list base - it helps them understand what best fits their needs, keeps them aware of innovations that might make your new product or service of greater help to them than their current solution, and keeps your company top-of-mind for your prospects.
And the time-limited offers not only provide a strong reason to buy now, but also keeps them on your mailing list longer for fear they might miss savings they want to take advantage of. In short order, once you begin building a bigger customer mailing list base with a mailing list it all becomes routine, and you'll automatically start adding as many people as you ethically can to your prospect lists.