I get perhaps the greatest increase in drawing skill in the waiting room — during those odd occasions when I draw something to pass the time because I am planted somewhere with nothing to do.  To avoid disturbing the other people who are waiting, I refrain from drawing them.  Instead I draw from photos I carry with me, or sometimes I draw from memory.  There is no pressure to have the drawing conform very much to anything and I let myself draw fast or slow, this element or that one, and I find that extremes of not caring often provide felicitous awakening of the senses, freeing the hand and the mind.  I learn from these episodes more and more to just toss down the line.