There are two things that have helped me build my lists.

One is offering your own product to somebody to include in their launch as a bonus.

When people buy the product, they're going to go to a page to download their stuff. To get yours requires that they opt-in and the product is delivered via autoresponder. Alternatively, you can provide a coupon and the product creator just includes a link to your order page where they enter their information and get the product for free via the coupon code. As long as they have to opt-in to get it, you're good. It's a win-win as the product creator gets to add a bonus and you get to build a list of people that you know actually bought something.

The other thing that's helped me quite a bit is launching WSO's. Sure, you have to sell your product cheap, but you'll build a list of buyers that way.