A well maintained bicycle is key. Oil the chain so that it's less likely to get hung up. It will have a smoother flow. Another could be in their clothing. Notice they usually wear good fitting clothing? I believe the more from fitting pants help clothing friction. Another way could be keeping the tires well inflated so that the tires aren't dragging. Rolling along smoothly on properly inflated tires seems like a must. Get a few more answers along with mine, so that you have a variety to choose from.
