When's the last time you purchased a product or service from a company with a nearly unusable website? How long do you stick with that company if a competitor offers a better user experience? Usability testing done by yourself or through third party services puts you in the shoes of the end user. This testing should answer questions about whether a user can easily find what they're looking for, if the content matches up with their expectations prior to clicking through, and if they are getting lost in an un-optimized website.
Start off by doing your own usability testing. Approach the site as though you're a first time customer with no idea about your company, product or services. Think about the way you navigate the sites of companies you use on a daily basis, and how your website's experience compares to those. Look at competitor sites and observe how easy or difficult it is to get around. Return to your site and keep track of every usability pain point you encounter.
You should also consider using a third party usability testing service to get truly unbiased impressions of your website. Ideally, use a testing service that allows you to specify a demographic so you see how your site is received by people who may be in your target audience. The end users go through the site, testing different website functions and giving you a usability rating and suggestions based on a rating metric.
Implement the recommended usability changes in your website and take your site through as many testing rounds as you feel necessary to truly optimize your user experience. Typically, going through a round or two gives you measurable change, especially if your website had poor navigation when you began testing.
To see the value of usability testing, look no further than eBay's navigation evolution or Amazon's development to today's design. Over the years, they have tweaked their navigation to make it as easy as possible for customers to find what they need.