Before going into your favorite software to design a banner you need to think about it for a bit, you need to think about what colors you might be using and what will be in that banner while taking size and contrast into consideration.
If the site that you're banner will appear on is white then you need to stay away from light colors, simply because the banner won't have a constant with the site and just blend it (causing people to look over the banner and not click on it).
You also need to know what is the size of the banner that you are creating.
The smaller the banner the less it can fit in there, so you'll need to be as creative as possible to get the attention of the person who is on the site and sees the banner.
Also, before designing the banner you need to find out what message should the banner bring, you need to find out what emotion the banner should have.
Imagine a banner that says, "A 21 year old kid makes over $5000 a week while sitting at home! Even the gurus are shocked! Find out how, now!" and a banner that says, "Create a customized toolbar for your website".
As you can tell, you had two different emotions and tones running through you as your read it and thought about it.
So make sure to find out the tone and emotion of the banner, that way you will connect a lot better with the people who'll be viewing the site your banner is on.