What are the essential things to know when creating a banner ad?

I've seen other posts that ask about creating banner ads for cheap, but none that focus on the actual elements that should be in a banner ad for a startup. What are some general tips for creating an effective ad banner? Colors, dimensions, animations or not, fonts, etc.

Colors and Dimensions are depending on the places where you are going to insert the banners. Colors are having different meanings and will create different feeling, so it is important to choose colors which fit the product/service the banner is advertising. E.g. Red is a warm color, which is standing for Love, Fire and so on. But it also could be seen as Blood.

Here are some links about Meanings of Colors:

http://www.squidoo.com/colorexpert http://crystal-cure.com/color-meanings.html http://www.smashingmagazine.com/2010/01/28/color-theory-for-designers-part-1-the-meaning-of-color/ If you search the web you will find a lot more.

To find good placing for your banner, see what other know about the best placing. E.g. Google shows which places are best for its AdSense Ads. What is good for AdSense is good for your own banners too (see https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=17954 for details).

Fonts should be good to read. If they are to big, it will be seen as shouting. Animation, well, I'm not a friend of this feature, but if it is down well, then why not. It must fit to the overall banner concept. An animated banner might get more attention.

When placing the banner, don't forget a link with a referer like "?referrer=siteofbanner_banner1" or the like. This referer should be seen in your Web-Analytics, thus you are able to see how it is performing.

A good headline is very essential for your banner. Also don't get to much text or to fancy graphics on it. It must have some harmony in it, but must stay catchy.

Last but not least, take care of the banner file size. If file size is to high, the banner will load slow, which nobody on the internet is a friend of.