EasyGuide.com

Useability Design Guide:

Most successful web sites are also easy to use. Think about this:
People are usually busy and don't have time to endlessly visit lots of web pages. When they do visit a page, they decide what to do in less that three seconds. If they can't figure out the site, what it's about, where to click, they hit the Back button, or (most often) Google Search, and they are gone.

How people REALLY USE a Website.
They quickly scan the page, looking for headlines, graphics, links, and anything that looks interesting, relevant or catches their attention - or next......

Make it about them.
Make your site all about "Users", your visitors. Give them useful information, in a straightforward manner. They don't necessarily have the same feelings, needs or wants as you do, so make sure it's all about them and their needs. It might be helpful to develop some personas.

E-Commerce websites
"Users" to the site will usually fall into the following groups:
1) They are ready to buy, have credit card in hand, and are shopping !
2) They are early in the sales process and are searching for more information
3) They are not potential buyers, just looking for information
4) They stumbled into your website by accident

Simple navigation.
Make your navigation scheme as simple as possible, and keep it the same throughout the site.
Keep it simple. Don't use mystery meat navigation. And, repeat your navigational links in both the content and on the bottom of each page. Give "Users" more than just one way to navigate through your website.

Make it obvious.
In three seconds, visitors should be able to determine:
A) What your site is on about
B) The navigation scheme and how it works
C) Where to find the information they want
D) Where to click next

For more information about how your website should be designed, read an interview with Steve Krug, Useability expert.