What advertisers have to decide is what they're looking to accomplish with online advertising, then modify their expectations accordingly. If you're looking to drive direct sales or visits through your online advertising activities, you should concentrate your efforts in a direct marketing mode. Your banners should have an immediate call to action. You should use email. You should look at contextual commerce links.
Success is harder to measure.
Building awareness can take a long time and requires repeat exposure. And because of their small size and tendency to get lost in the noise of a site, banners might not be the right vehicles anyway. Sponsorships, advertorials, and larger-format ads are probably better ways of building knowledge of your company over the long term. If you're going direct response, measure clicks. The online advertising industry was able to gain credibility and customers early on by selling the measurability of the medium while neglecting its ability to ensure brand recognition.