There are endless strategies, tools, and tactics you can leverage here. I personally would start with any low hanging fruit possible. Not knowing your business or having any context makes it more difficult.

I personally love Optinmonster. You can create a series of popups that can be positioned around your website to optimize and encourage people to optin. Focus on the pages with the highest traffic first. I would avoid instant pop-ups as these are just annoying so use delayed popups to ensure the "prospect" is interested. As well exit intent pop-ups are great as well.

Next, I would focus on your copy throughout the website while leveraging A/B testing to see how you can improve your optin copy.

Once those are optimized then I would focus on two areas.
1. Retargeting Ads
2. JV partnerships
3. Guest Posting
4. Paid Media

More than happy to review this with you and share these ideas in greater depth if you're interested.