After conducting hundreds of usability tests, here's what I've learned:

Usability testing should begin long before the site is built

You need to make sure the website organization and terminology make sense to the users. It is also important to understand what users expect to be able to do/find on your website - and what they consider most important. Understanding these issues before coding begins will save a lot of time later.
Interviews in which users sort information in ways that are logical to them can help you understand how to organize your information.
Interviews can also help you understand the terminology that is familiar to users. What do they call the items or ideas featured on your website?
- One client used a term on their website to lead to an extensive area dealing with international information. They were certain that all users understood this term. Their very experienced users did, but newer users (the bulk of the user population) did not. Early research on terminology could have helped them avoid this problem.

Consider how your marketing standards should be implemented on your website.

One client has a beautiful TV and print advertising campaign with stunning, evocative photography and copy that focuses on the product use experience rather than the product itself. There are lots of emotional words and adjectives. They brought this same approach to their website - which frustrated and confused users.
- The many adjectives created long paragraphs users did not read.
- The focus on experience rather than a straight-forward "what it is and what it does" often left users confused even after they had read a product description.
When planning your website, think about how to make it fit in with other elements of your marketing plan, but still be usable.

Avoid long paragraphs - especially on your home page.

Users are in "scan mode" - looking for pictures, words and phrases to lead them to the next step.
They will read a few short bullet points, but,at best, will "skim" paragraphs.
Many also will not scroll below the "fold" - so if your text requires them to scroll, they probably will not read it.
Keep your home page as clean and simple as possible. If some information is critical, use bullet points.

Ask the questions you need to ask to provide information - and no more

A client's website provided information for users based on some parameters they input. However, earlier a decision had been made to ask several other demographic questions too, "while we're at it."
These additional questions led many users to quit the website. As one respondent said "It's like a date - you barely know me, I haven't made any commitment to you yet, and you start asking me all these personal questions - too much - I'm outta here."

When a user takes an action, make sure there is an obvious confirmation

On one website, if there was an error when a user filled out a form, the error appeared at the top of the page. Most users did not notice it became very frustrated repeatedly clicking "submit" which was at the bottom of the page.
Confirmation should occur close to where the user is working. On one website, the user input information on the left side of the page, but after clicking "submit" a fairly subtle change occured in the center of the page. Most did not notice it and could not understand why "nothting happened."