Try increasing your speed or incline so that you feel challenged, for at least part of your workout. Interval training, where you run hard for a period of time and then cool down for another interval, is a good way to push the pace without pushing it for the entire run. You can do interval training once or twice a week (never two days in a row). Try some of these workouts:
Of course, when you're running hard on the treadmill, it's still important to make sure you're doing a proper warm-up and cool-down.
More: Tips for Running Faster