Use keywords in your titles and descriptions

Research indicates that listings in which keywords are included in both title and description receive a 50% higher CTR than listings which have the keyword in the title only. The reason? In most search engines, keywords are bolded.

That means a listing that looks like this:

PPC Ad Copy
Get PPC ad copy advice from the experts.
www.adcopyexperts.com

is likely to receive a higher click through rate than a listing that looks like this:

PPC Ad Copy
Get PPC ad copy advice from the experts.
www.adcopyexperts.com

I recommend: Yahoo!'s Guide to Getting More Traffic: http://searchmarketing.yahoo.com/rc/srch/intro101.php
The Click Z Network: http://www.clickz.com (An excellent article on this site is "PPC Search Copywriting Strategies" http://www.clickz.com/showPage.html?page=3445361) 

Advertise low prices

If given a choice between similar ads, a user is more likely going to click on an ad that advertises the lowest price. If one of your main selling points is your low prices, then go ahead and put that price in your title. But, if you can't outbid your competition, don't include the price in your ad copy.

I recommend: Marketing Experiments Journal, especially the must-read article "PPC Ad Copy Tested."

Include a call to action

Here's a fact: people actually like being told what to do. Think about your landing page, and what you want to accomplish with your ad. Do you want to sell a product? Do you want users to download a trial version of software? There should be a specific action you want users to take, and you should tell them to take it.

Examples:

Buy our Web-based software program today.
Download whitepaper.
Get a quote.
Apply now!

I recommend: Google's AdWords Learning Center (Writing Targeted Ad Text)
http://www.google.com/adwords/learningcenter/text/18779.html

Think twice before using your company name in your listing

Research shows that using your company name in your PPC listing only helps your CTR if your company is already a well-known brand. In other words, PPC advertising is not the best way to increase brand awareness. Additionally, the advice from Google AdWords indicates that your company title is already considered part of your ad copy because of your displayed URL. Each search engine has a maximum character for ads. Using your company name in your ad copy when it is already in your display URL is just wasting precious space.

I recommend: 
Google AdWords Learning Center (Creating a Specific Ad Title):
http://www.google.com/adwords/learningcenter/text/18779.html#18782

Avoid superlatives and excessive punctuation

All search engines have Editorial Guidelines that advertisers must adhere to. While it may seem like these "rules" are in place just to follow standard and proper grammar, there are bigger issues at hand. Users are more likely to click on an ad that looks professional.

Bad:
The Perfect Ad Copy for YOU!!!
We will write the best ad copy on the web 4 U - FREE!!!
www.adcopyexperts.com

Good:
Customized PPC Ad Copy
Our team of experts write customized PPC ad copy for your search engine campaigns. Download a whitepaper today.
www.adcopyexperts.com

I recommend: Microsoft Digital Advertising Solutions.  A great article is "Write Ad Copy that Resonates with Your Target Audience."

Customize your ads for each search engine

With the trend towards vertical search, make sure your ads are tailored for different audiences. Sometimes a listing that performs well on one search engine will return less-than-stellar results on a different search engine.

For instance, Business.com offers advertisers "multi-links," which are smaller text links below ads. Studies have shown that listings utilizing the multi-link feature have a higher CTR rate than listings without. Multi-links are a great way to target specific landing pages of your site.

Examples:
The "About Us" page
A demo feature
A paper download

I recommend: Business.com Advertising Center http://www.business.com/info/knowledge_center/optimization_tips.asp