In this article on the Email Experience Council's blog, Lisa Harmon explains how to make your call-to-action stand out:

1. Stay focused.
Design the entire message to direct recipients towards the CTA. Don’t distract them with too many equally-weighted links and offers. Select imagery that draws the eye toward the point of conversion. Make the path appealing and clear, and make sure that it extends beyond the email itself to the landing experience.

2. Keep it direct and clear.
It’s fun to write clever copy, but make sure that even the quirkiest wording is to the point. 

3. Make sure it’s above the fold.
Keep the CTA above “the fold,” or in the part of the message that’s visible without any scrolling. While the fold location can be hard to predict with all the varying preview panes and computer monitors out there, put your CTA up top where it gets the attention it deserves.

4. Make the CTA stand out visually!
Keeping it above the fold is a good start, but go further. Make your CTAs stand out visually. Try using HTML buttons as opposed to text links. You’ll grab more eyes that way and generate a higher CTR. For more on buttons, check out Lisa's article on “The Bulletproof Button”.

Read the full article (including examples) here.