Look to build a long-tail keywords list (a list of keywords that have enough searches on Google, but do not have too much competition), write mailing list relevant ads, and direct traffic to your opt-in page. Giveaway events are a great way to quickly build a mailing list. You have some options too - you can either join an existing giveaway event as a contributor, or you can set up your own and both organize it and contribute to it.
Contributors provide digital items - including ebooks, audio files, manuals and similar products, for free download to individuals who attend mailing list the giveaway event and supply their email addresses. When someone enters their email address, they are given access to the downloads for the products of their choice. Contributors then quickly build a mailing list of names from the people who request the free product downloads.
The first thing you need is the main web page that describes your event. It is on this page that site visitors would know that if they sign up for mailing list the giveaway event for free, they will gain access to many pages of free products from various internet marketers and contributors. As the organizer of the event - you will benefit from gaining the email address of every single person who wants to view the products available for free download!