First of all, you can start by getting the names and the email address of those people who are buying at your site; you can give them an option, after mailing list they finish their buying, to give you the name and the email so that you can send them free offers. These offered products should be relevant to your main products of your website, then you will be sure that the offer will be attractive.
The most important thing in your offer is its value for which your visitor will accept to give you his name and his email. At the same time, it is not enough to attract mailing list people by putting a link and a simple download on your site. But, you should try to choose a noticeable or distinguished place in which you will put your download form. It is also desirable of you to accord a page to it, in addition, you may put a link to it in every other page in your website.
Finally, you should give some information mailing list about the download you offer to your visitors, as a promotion to it and also to give them a chance to realize the value of it. This mailing list for sure will help attracting them to download it and in turn pass to mailing list you their personal details aiming to get any new products that you will send to them.