I hope that most of you at least make a small effort to improve your emails by A/B testing. It’s really not hard – whenever you’re sending out an email to a decent number of people, split the list in half, change something significant on one of the emails, and see which of them gets the best response.
One big test which you might consider is comparing normal “HTML” emails to plain text ones. But only do this if you’re prepared to follow through with switching to plain text emails if they “win”. Because it’s possible they just might.