Sketching portraits

Looked at many sketches from various sketchers' blogs around the world, linked from Urban Sketchers (www.urbansketchers.com) and am really amazed by their ability to sketch. My personal favourite is this singaporean, Don Low(http://www.donlowillustration.com/blog/) with his unique style. My goal is to be able to sketch portraits smoothly. As much as i want to blame the people for moving too frequently or having faces not distinct enough for sketching, it really is my lack of skill that is the cause. Hope that one day i can sketch people like i sketch buildings and still life.