Jakob Nielsen in 1997 wrote an essay on why every amateur web designer should do usability testing, at least with a couple of people.
Sit down with someone and watch them try to do something with your web site.
In 2000 he did a statistical analysis on why watching 5 users is enough to get almost all of the usability testing problems out of the way.
This is pretty simple stuff. So ask yourself: when is the last time you watched a person try to use your site? How much effort would it take to actually get with a few people and ask them to use your site? What kind of dividends would it pay if 90% of your usability problems could be detected with a simple test like this?
Why is it that the simplest things that pay the biggest dividends so often get overlooked in our day to day business operations? If you are too busy to talk to your customers or watch them try to use your site, then something is wrong with your business culture. Fix it.