Good design/Punching quotes of ad if you're advertiser and good content on your site if you're publisher
look at what top competitor ads are on the words you are bidding on. copy and improve their ads to get the upper hand
ratio of people clicks on your ads over number of times your ads appear. the higher is better since people attracted to your ads
Model off the top 3 ads for your competing keyword... and try and better it even more... Use "power" words, or words that attract people to read... Also helps if you have a nice URL that helps with the "CTA"... Honestly though, you're better off to buy a book on this subject, there's so much to learn about it... I could probably write a book on it myself! James
Are you running your ads on the content network and search? Turning off content should help increase your CTR a bit.
To increase your CTR, try to write a better ad. Imagine if you are the one looking for business or product. What ad will entice you to click on the ad.