We've done a great job creating solid content and driving traffic (via social and SEO) based on those content. However all these visits have been just visits - we haven't been able to convert them into signups.
Give something of value away.
In my own experience working to grow my email list over the past 6 months and having tested a number of different forms and types of forms, the places I give away a free digital download have a conversion rate 4x higher than the places where I try to entice people to sign-up to be subscribed to my newsletter or be updated when a post, i.e. app list, is updated.
I'm seeing that giving away a short course or tool is producing better results than an eBook, for example.
If you're using WordPress, Thrive Themes has some fantastic conversion-focused themes and plugins. I hesitated for a while to pull the trigger, however did about a month ago and haven't looked back!
Hope that helps and good luck!