Your subject line and preheader text work together to get people to open your email. Use these to get your point across at an emotional level. Create excitement or urgency that makes them anxious to find out more.
We're not reading our inboxes anymore. Readers will skim it for the important bits. (I've done this.) Imagine your marketing email without any text at all. Create a story or stir emotions just with your images and see what happens.
Consider including a face that looks toward your call to action. Close-ups work well, and eye-tracking studies show we'll look where they're looking.
Let's face it, every audience is different, so you have to run tests to discover how something like color affects response. Color studies help you figure out what to test with your readers. You never know what color might be the key to better engagement.