When I sketch, I often fall into the habit of attempting to create a perfect image - or as John calls it, a pretty picture. However, he encourages sketchers to go for quantity instead of quality. John says:
"Enabling yourself to make as many pictures as possible opens doors. Each drawing becomes less important to the ego and one drawing invites you to the next. Sketches without judgement help you maintain your focus and embed you in the moment. This is where journalers experience flow, a state of utter concentration, fascination, and connection. The sketcher often loses all sense of time. The experience motivates you to continue and do it again."
Take a few minutes to read the complete post. Then pick up that pencil and Go for Quantity. Later, check out the rest of John's site for some great tutorials.