How long do you think people look at your banner for? According to results from Stanford/Poynter research, just one second. That's it. If your banner's creative takes longer than a second to run, you should test shorter-running creative.
Stanford and The Poynter Institute; both non-profits which have nothing to gain from the online ad industry - teamed up to measure how consumers use online news in 1997, and have been improving the study ever since then. At first they videotaped people surfing, now subjects are hooked up with a head harness that tracks their eyeball movements (no joke, see photo
Eyeball tracking studies of this sort were used in the cataloguing industry in the 1980s to determine how catalog shoppers browse. One of their biggest findings at the time was that people look at big pictures first (especially of people) and then their glance goes to the right (unless they are Israelis reading a language that goes in the other direction). This is why to this day catalog layouts generally have the biggest photos at the left and the copy to the right of the page, instead of vice versa.