Color: Is the color of your call-to-action the same color as an active item on the menu? Try making your CTA unique from other elements on the page.

Copy: Use personalization. Instead of 'Click Here,' add context: 'Register Now,' 'Download Whitepaper,' or 'Listen to the MP3′. Almost never use 'submit.'

Image: Consider testing icons on your CTAs that indicate what action the visitor is about to perform or the type of offer they will receive (a whitepaper, an mp3, a newsletter, etc.).