When you open up your software of choice to do the banner you need to be really professional about it.
You need to realize that your banner represents you and the website or campaign you are promoting, this means that you need to present it in the best possible manner.
It means that you need to be extra careful about the spelling and the grammar of the banner.
You also need to watch out for font sizes, styles and colors.
For instance have you ever noticed how products for dish washing, and floor washing and cleaning in general always come in a color of blue? Well that is because we as humans over a period of time were programmed to think that way, we were programmed to relate certain colors to certain products because of the way those colors are represented in the real world.
Blue will represent water, while brown will represent the rich soil, green will represent the ecosystem while purple will represent magic.
As you can see, all those colors already have a meaning behind them even before they are implemented into banner ad design and graphic design in general.
By having this kind of information before you start any project is really good.
You can connect more with the people who will be viewing it, making your project more interesting to look at and a lot more impressive, but on the subconscious level.
This mean that a lot of the times they will like your project but sometimes they wouldn't be able to explain why they do.