Keep the filesize small

You must aim to keep the size of the banner as low as possible. If the banner is large it will take more time to download, and the visitor may have scrolled down the page or moved on to the next page, which means your banner will not be seen. Make sure your file stays below 10K, or 15K at the absolute most.

Use as few colours as possible, and consider using compression to further reduce the filesize.