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!