Timing is everything. Doing user research (usability testing) after the redesign is too late in the process. Find what your target audience needs by conducting user interviews early in the project. Avoid learning what you have done wrong by the time is too late to fix it.
Before re-designing an existing site or beginning the design or development of a new site, determine who is going to be using the site and what they will be trying to do there. Optimize the site organization and user flows for easy and frustration-free accomplishment of their primary tasks/goals in using the site.