Thanks for getting in touch through the forums, it's Shauna here from the guru team.
It's really great that you have established what works well for your business in terms of marketing.
We have a great blog post on growing your email list here- https://www.shopify.com/blog/13042257-how-to-build-an-email-list-that-builds-your-ecommerce-business It's packed full of tips and ideas which I'm sure can help you out. :)
The email pop up box app is great! It helps get email addresses for all that traffic coming to your site.https://apps.shopify.com/email-pirate
Hope that helps!
We use MailMunch to create a popup box and a top bar to capture emails. It's been working great! They have several types of capture forms that you can use, including exit popups. I have integrated it with MailChimp.
Check it out at: https://apps.shopify.com/mailmunch
LJS is correct. I think increasing an e-mail list is a function of traffic as well. And I think your subscription rate has to be greater than your e-mail unsubscribes, bounces, complaints, etc.
But ultimately, more traffic is key. We can all use more traffic. Even Jeff Bezos asks for more!
You can use Facebook ads and target people who follow competitors. Send them to a custom landing page to sign up with your incentives. Here's an example I created for my shop: http://worldabomination.com/pages/world-abomination-gamer-tee-giveaway-and-subscription