What Every Email Marketer Needs to Know...
Write subject lines that multitask. Every subject line must accomplish a number of tasks: telling subscribers what they get when they open the message; making the offer sound irresistible; and doing it in a relatable, conversational voice. Do not use symbols of any kind (i.e. ! - ? - $).
Create a sense of urgency. Without a compelling reason to do what you're asking the reader to do—right this second—they'll go back to their inbox and unsubscribe. Urgent, action-oriented words are more successful than words such as 'free' in inspiring your subscribers to take action.
Keep a tight focus on your offer. Stick to just one offer with a clear path to action—unfettered by the distraction of extraneous sidebars, content, links, or discounts. A call to action button should be centered on the page—right in the reader's main area of interest.
In short... It all comes down to a little push. No single factor guarantees a successful call to action—but if you get each element just right, subscribers won't even consider the option of inaction.