Numbers always speak louder than words.
If you can say that there are twice as many customers who are capable of finding and ordering products after a redesign, then your message carries much more weight than if you give a speech about the improved navigation system and streamlined checkout process. It is useful to track the evolution in the usability metrics over time. Keep measuring this, and you will drive up both the quality of individual designs and the organization's ability to deliver quality effectively and predictably. Competitive measurements are often very convincing. I know of one e-commerce project that was doing fairly poorly.