When your done with the sales copy and your banner is made, test, test, and test some more. Once you've got the perfect banner working, design another one. Sooner or later even the best banner runs out of steam. It's generally accepted that once someone sees a banner 3 or 4 times, the likelihood of a click-through drops off considerably, depending on your ad. When testing, always try the new banner on the same site or on the same network, so you are indeed comparing apples to apples.
Testing is an ongoing, dynamic process and is necessary for a successful ad campaign.
Review your statistics pages on a daily basis and be prepared to act quickly when results begin to lag. Remember that the most critical element of your banner is the sales copy. Once a successful banner has been created, tweaking and modifying text and graphics properties can keep it alive and producing strong click-throughs for many months. The words below are powerful. When used properly, they can grab a person's attention with a whisper.