Here is list of guidelines and observations which you should follow when designing banners. Some of this points can overlap. I gathered them from various news group postings. Ads should animate and loop. Non-animated gifs don't get noticed as much.
Bright colors perform better than subtle colors.
White is the lowest performance background color, Bright pastels and florescent perform best. Banners that have a call to action work best. Banners that make an explicit value proposition work best. Fewer words of copy are better. Banners should communicate message clearly.