Of course, if you're going the paid ad route, you could also use Facebook and Google re-targeting to keep that awareness and interest level high. For example, if you've ever noticed after leaving a particular website, that you begin to see their ad everywhere, there's a particular reason for that. Especially if they've already entered your sales funnel, this is a very powerful way to get them to act.
The effort you put into building your email list is one of the best investments you can make online. Having access to the inboxes of targeted prospects means you can continue to build and nurture relationships over time, and become a trusted source of valuable industry knowledge. Then, when it’s time for your prospects to buy, you’ll be the first one who comes to mind.