Find an amazing image! I normally find one that would create contrast for better readability. One site I use is "Unsplash" https://unsplash.com/.
Have a great catchy call to action like, "Subscribe Now" or "Sign up for 10% off". You want to create some urgency for the user.
Placement of you call to action is key! Short, simple and sweet is the best approach. Please see this example below from Indystructible Designs:
