CTR

CTR or Click Through Rate is the amount of clicks an add gets compared to the times it has been showed. A lot of research has been done on what ads generate the highest click through rate. This is important as most publisher networks still charge per 1000 exposures (times a add is displayed).
One little fact the automotive branch uses, is that cars viewed from the low angles focused on the left or right front of the car generate a higher CTR.
There are many other techniques that boost CTR though and it pays to do some research on this topic.

One way to do this when using Google Adwords or Yahoo Search Marketing is to use several campaigns at once and compare their CTR. Keep the campaign with the highest CTR and copy it to a new campaign. Now write a new campaign text and let the three campaigns run for a while. Let some time pass and again keep the one with the highest CTR and begin the process again.

The good thing about this system is that you will only put 33% of your advertising budget on the line for finding new campaign strategies.