There are multiple ways to improve your delivery/open rates. There is not just one thing that you can do but there are a variety of factors that may help.
The first part of doing a suggest campaign is using an effective subject line. We suggest doing multiple tests in chunks to see which subject line is the most effective. For example, if you had a list of 100,000 you can send 50,000 to the first subject line and 50,000 to the next subject line and see which subject line is the most effective.