Discover the exact cut-off date you should have in place for people who don't open their email... If they don't interact with you, you shouldn't be wasting your deliverability on them! So, cut them out!
This one thing alone will drastically reduce your email marketing costs AND give you a better, more responsive list; no matter how hard you worked to get them in the first place!
(You're also going to discover how to 're-activate' a lot of them with one, solid email message!)