Co-registration is not for everyone, but in some niches it can work quite well. Basically, you find partners in your niche who will place your opt-in box on their think you page, and you will put their opt-in box on your thank you page.
This can work well since these are proven customers in the niche, but you do have to be careful. It has to be done in a way that is not tricky to anyone so that people know what they are really signing up for. You'll gain a lot more trust if people realize that they are signing up for your list.
Insist on making it crystal clear what your potential subscribers are signing up for.