It must be something that is valuable to the customer, and very inexpensive for you. Information is the best reward. Give them access to your technical data, to your archives, to your new product specifications, to your future forecasts. Banners are one of the cheapest and most cost efficient ways to advertise web sites. An impression is when a viewer clicks on to a website that contains your banner.
Banners are much more cost effective than the same money spent in a newspaper, magazine or TV ad.
Because a magazine reader is, by definition, not at her computer while she is reading. She may not even own a computer. When she clicks on a web site that contains your banner, she not only owns a computer, she is using it at the moment, she is on the internet, and can easily click on the banner to jump to your site. By using banner advertising, you let the readers self select themselves into likely prospects. I have heard people say that banners are dead.