Do you have tips/recommendations to optimize engagement and click-through rate?

I'd really like to see what I can do to optimize engagement and click through rate on our current email newsletter. I'm seeing that our lead magnet, it has a great open rate. It's an 80% open rate, 60% click through, and then subsequent emails out the gate, you know, it's about a 20/25% open and 6% click through rate. 

What recommendations do you have to really kind of help boost that sort of thing?