Let's say you have a product or service that you sell offline in the real world. You put your Web site address on every vehicle, you post it in every shop window, you put it on your packaging, etc. You make it very easy for your existing customers to come to visit your Web site. You may even give them an incentive to do so. Major couriers do it by offering online tracking of shipments.
Major airlines do it by offering online reservations.
Major food companies have done a terrible job so far, but some offer recipes, etc. for visitors. You engage your customer in a dialogue, you show the customer additional value from the visit to the Web site. At that point, you are ready to request that the customer raise his or her hand and give you permission to send E-mail about certain things that may interest the customer. The more options you can offer that fit the customer's needs, the more the customer will likely want to sign up. When you have the dialogue going with the customer, you can then sell other things within your brand.