As for the teqhnique - I carry small sketch books, and a pencil/pen set with me all the time. Fill up some pages with small gestural marks. Vary the pressure of the pen to get a range of value. Vary the space between marks. Dance with your pen on the surface of the paper moving in tightly, then move back out to a looser weave of marks.

On an image like this, I sketch out the main shapes and then develop the dark areas first. This allows for a warm up to get the feel for the lighter pressure that is needed for the lighter grays.