Don't just list your job responsibilities in a resume. Think of each one in terms of an accomplishment that can be qualified.
A very common resume mistake that many people make is to list their professional experience by company and then list their perspective responsibilities under each one. If you think about it, all that's telling a potential employer is what you were expected to do at your previous jobs—it doesn't necessarily mean that you actually did any of them or that you were good at any of them.
For example, keeping your company's Web site up might be one of your duties, but unless you can express that as an accomplishment in terms of downtime percentages, then it really doesn't have much punch. Try to stay away from the words, Duties or Responsible for...
You can list what you were in charge of in past jobs but do it in terms of what was accomplished. Before you list a responsibility, think of how you can rephrase it to reflect how your action saved the company money or time, or how it expanded the business or attracted new customers.
Think of your efforts in terms of how they helped your organization overcome a problem. Did you leave your employers better off than before you came along? How did you do that?
So, if one of your "duties" is optimizing the network for customers, state it as, "Maintain strict adherence to customer turnaround times, consistently achieving on-time completion of system repairs." Don't just state the responsibility. Be specific about how well you did it.
Try to qualify statements whenever you can with figures or measurements.
(To help you keep track of your accomplishments at each job, here is a free Accomplishments Tracking template you can download from TechRepublic.)