Purpose of a display ad
To create a focal point for the eye on the page so that the reader can easily absorb the most pertinent information (and only the most pertinent information).
1) Get an artist. A common misconception about display ads is that anyone competent with a computer can whip up an ad. Untrue.
2) Less is more. Another common misconception about display ads is that you take a box and just fill it with words and sentences. Also untrue.
Here is an example of a bad ad.
Here is an example of a good ad.