Keep the file size as small as possible. Under 10 KB is optimum. Include your domain name on the ad if it is easy to remember ie. That way, they can always visit your site at a later time. Add animation, it gets people's attention.
Bright colors seem to work well.
CPM - Cost per thousand impressions or page views. When a visitor clicks his or her mouse on a banner ad, the visitor is automatically transported to the advertiser's Web page. The percentage of visitors who click on a banner ad. The number of people who click on a banner ad and get to the advertiser's Web site. Reprinted from Web Marketing Today, Issue 16, June 22, 1996.