Good question Ernest. Sketching, in its simplest form is a quick process, so I concentrate on capturing the basic lines of a position with as few lines as possible.
In situations where there is a captive audience such as airports or restaurants, people are sitting in one place for varying periods of time. They can, at any moment, move, so you have to be quick to capture the pose and not concentrate on details too much. Put down stick figures to capture human position, the build on it from there if your subject sticks around. You can fill details in from memory or let the surrounding environment become more detailed to draw the eye away from the sketchier subject.
Practice is very much the key. The more you draw, especially from life, the quicker you are able to establish shapes and values. Sketches are not meant to be finished drawings, but simply snapshots of time.
Study the skeletons of the individual or animal and imagine how it moves. That will give you the position. Then adding 'meat' to it becomes easier, provided the person stays around long enough to capture some detail. Use minimal lines to capture the essentials that help the viewer see what you see.
Try some quick sketches from television programs or movies. See if you can get just the body positions in stick figure form. Allow yourself 30 seconds to do this. It doesn't seem like a lot, but once you get used to it, you can capture a lot in 30 seconds. Draw in restaurants, waiting rooms, hotel lobbies, airports, train stations, airplanes - anywhere that people will perch for a few minutes and become absorbed in their own world.
There is no real advice to give, except to not be self conscious when drawing in public, be unobtrusive - I keep my drawings in a hardcover spiral book so it doesn't scream ' I'm going to draw you' if I take it out at Starbucks. I look at people but past them, so they and I are not uncomfortable, though I have never had unfavourable comments if someone knows I'm drawing them, they're usually just interested in the process.
And practice. Constantly. Daily. Forever.