So far the largest mailing list I've built up consisted of about 100 subscribers which is pretty lousy. Some people sell systems that are supposed to reveal the secrets of getting tons opt-in subscribers. As far as I can see there are no secrets.
The only two secrets - is a great squeeze page and loads of traffic. That is what you need to get ppl to subscribe. To keep them subscribed is another story
As said before you need a very good squeeze page and generate traffic to that page from different sources like article marketing, ezine advertising and ppc to name a few. Then the most important thing is the level of trust. The better this is the more you will sell. You also need to become very good at email copy for example subject lines that get opened etc... Hope that helps.
One powerful way to build your own opt-in list is to transform an affiliate link into a list-building resource. As an affiliate, there are two things I'm sure you do. The first one is to cloak your link. The other one is to direct your traffic to a squeeze page before you send them on to the vendor's site. If you use cloaked re-redirection to cloak the link, the two actions I've mentioned can be combined. What I mean is that you can set-up an in-transit opt-in form which is displayed for a set number of seconds when your cloaked affiliate link is clicked. Simply make available a valuable free gift in exchange for the first name and primary email address. As long as the free gift is seen as complementary to whatever product or service is on offer, a third of visitors who pass through should sign up. The homepage of solavar's profile has a fuller article explaining it all.
create a unique report that no one has and offer it and watch your list grow - for example no one talks about clickriver for ppc marketing and I made a report on it and I get about 7-10 new emails a day http://www.ianfernando.com/clickriver I simply put valuable information teasing the user and they want to know more so they give me their email address - hence squeeze page
Track your ops, and find out when they unsubscribe on average. Then move it back 1-3 emails, and sell your plan by that point and time to the reader. Anyone that falls past that point should be removed from your list as they are not customers only failed prospects.