Paid subscription sites, what're your go-to growth hacks to increase your free newsletter readers?

Crew, we have a newsletter/media biz doing 5 figs/month that grew from a free newsletter to a paid product: premium/paid subscription membership site. We rely heavily on email marketing and nurturing for converting free to paid, but feel like have tried everything to grow the free subs. What are some cool free newsletter sub growth hacks?? Any help / suggestions appreciated and will try to return favor!