If you're into Pay per Click as a means of marketing, whether you be an affiliate or a merchant, you'd be aware of the critical importance of ad copy. For those new to Internet Marketing (or IM), ad copy is the ad headline and the 2 lines of description you find underneath the ad headline and above the destination URL.

Let me say outright that the most important part of your ad is the headline, but if you don't have 2 killer lines of ad description, you may just lose those people who actually bother to read all of your ad. Also keep in mind that the advice here can, in many instances, be translated into habits that enhance your headline as well.
Ok, let's begin........

1. Include a Price Qualifier.

There are some instances where you want to entice people who are looking for freebies. Mainly if the destination does in fact have something complimentary (usually a teaser or trial) on offer. In most other instances, you want to discourage these type of internet users. After all, click through rate is important, but sales are the key.

Putting the price of the item in the ad description is a great idea when you are promoting a specific item that the user is looking for. This works especially well in retail. For example:

iPod Touch 8 GB $170

You don't really want someone who is not wanting to buy an iPod OR not willing to cough up the $170 clicking on your ad. If you price qualify the prospective visitors to the offer, your click through rate may go down, but your conversions will increase.

A price qualifier doesn't work as well when someone is looking for something more general, or, for information or a service. You wouldn't run an ad for meditation classes which said:

Meditaton Classes
$25 per half hour

You would put something like:

Meditation Classes
Discover Your Peace

You would try to pre-sell the idea BEFORE showing them the price.

2. Encourage Action

"Subscribe","Read","Download", "Now","Sale Ends Tomorrow". If you can invoke a sense of urgency or action in the user, they will be more likely to feel compelled to at least click through and make sure that they don't miss an excellent offer.

3. Make them need to do something

This is similar to number 2, but makes things a little more personal.

For example, your could change:

Download Our Free Report

You Need to Read our Free Report

4. Don't finish the sentence.

Adding small dots to the end of a phrase implies that there is more information to be had if the user goes ahead and clicks the ad. It's kind of hard to resist.

You Need to Read Our Free Report
Before You Sign Any Contract...

5. Tell the user the benefits.

You are giving them something that they will gain by clicking through to your ad. Basically, the user is always looking at what's in it for them, so why should they click YOUR ad and not the one next to yours?
You can sometimes get away with exaggerating here, though obviously not something obnoxious like

Become a Billionaire in 2 Weeks

Something more along the lines of

Gain 20lbs of Muscle in 3 months

Study For Your Diploma Online

All of these work on quite subtle levels. They are not things that jump out at you. The reason they succeed is that the user is basically scanning, then reacting to the ads, not analyzing them.