The most obvious method is via the company website. Indeed for B2B, opting in for email marketing should be one of the main aims of the site and only slightly less for a customers based one. Even a simple sales-base site should be seen as a permanent source of addresses for email marketing.
Incentives should be present at all times to capture email addresses: offers and special benefits for those willing to tick the box should be designed into the structure. Conversion of visitors to opt-ins should be one of the major pointers to how successful the site is.
The search parameters used by those coming to your site can be a useful guideline as to what to include. If “home dog grooming” comes up time and again then offers of clippers should be on the first page they come to.
There are many other methods of drawing potential subscribers to your site, such as a competition or other offer publicised offline which has a requirement to log onto the site to enter or to claim the prize. Other considerations are an online advert in, for instance, a third party site or newsletter or even a rented list, both of which ask the reader to click through to a microsite where they will be tempted to sign up to your list.
A viral campaign, such as the preview of the next meerkat advert, can be a valuable source. A customer sends the video link to a friend or colleague who will log on and be presented with an opportunity to opt in to your email list. Care should be taken to ensure the medium does not overcome the message.