Advisable to practice in memory enhancement by sketching a working person. Choose the most typical motion (a key!) from series of repetitive ones. Besides that, reproduce from memory all interesting you have seen for a day – specific faces, typical figures, poses, groups, etc.
You should sketch from imagination. After continuous sketching a half-face head, try to draw it in 3/4 view, in the front, in any rotation and tilt.
Sketches from memory and from imagination are based on acquirements and they are quite creative.
Artist should reach maximum expression by minimum techniques. You can catch and fix moving pictures of life only with fast working hand and good memory. Remember, imagine and then draw if there is no time to sketch now.
In most cases models can't keep right pose and expression for a long time. After a while their poses lose vitality and expression required for current work. In this case ability to come back to the initial impression is indispensable way to give the image true expression.