Make sure it's contend that relates to their interests, with call to actions for them to engage with. (leadpropeller.com)
I do have a question for you: Are there are restrictions for call to actions in my ebook? (jakonrath.blogspot.ru)
One thing you can count on with calls to action is that testing is never done if you're truly committed to optimizing your results and the return on your marketing investments. (entrepreneur.com)