Practice, practice, practice. I am trying to draw and sketch as much as possible at the moment. The beauty of working in a sketchbook means that I don't have to worry about pages being finished or perfect - I can just play with whatever takes my fancy. Some of my drawings are from life, others from photographs. I am getting better at showing buildings, but I still find people really difficult.
I guess that's where the practice comes in, but I am still not brave enough to post my pictures of people online! Once they start looking like the person they were meant to be, perhaps.