To color on not to color:

The colors choice that you make for your ad graphic is very important as it can work for you or against you. You'll have to limit your palette to keep file size low and stay away from dark text on a dark background. Colors such a bright red and yellow tend to do better than drab colors like brown and gray.

Don't keep beating a dead banner:

The forementioned study also concluded that ad banners effectiveness increased noticiably when banners were frequently changed. You banner's response rates usually decrease by second to third time someone sees it. We recommend that you rotate several versions of your banners every few weeks.

KeyWords & Experimentation :

While words "Click Here" and "Free" are not magic bullets, they do often help a banner get a get better response rates. Of course, if you have nothing free to offer, don't use word "Free". We don't do "Trick" banners.

The bottom line is that making effective ad graphics can be very involved - experiment , keep good performance stats, enjoy creative process.