Your brain is divided into two sections: the left hemisphere, which handles such things as logic and speech, and the right hemisphere, which by contrast is visual and creative. Right brain drawing, a term popularized by the book "Drawing on the Right Side of Your Brain" by Betty Edwards, refers to the way that drawing can help you flex the sometimes-ignored right hemisphere. You can achieve this through the use of a handful of simple exercises.
Print out the accompanying picture. It's one half of the classic optical illusion in which the image either represents a vase or two faces. Trace a pencil over it which announcing out loud what part of the face you're tracing over: "the forehead ... the eye ... the nose ..." and so on. When you're finished, immediately start filling in the blank side of the picture, a mirror of the first.
Are you done yet? How did you feel when you tried to draw the second half of the picture? Most people experience confusion or a temporary brain freeze when they start. This is because during the first half of the exercise, when you trace the half of the picture that's already there, you're exercising the left hemisphere of your brain: recognizing a pattern and identifying each part verbally. When you start to draw the second half of the picture, your brain is forced to switch to the right hemisphere, causing a temporary adjustment period.
Find any drawn portrait of anyone, print it out and turn it upside down. On a blank sheet of paper, draw your own version of the same portrait, also upside down. Don't take too long; give yourself 15 minutes or less.
When you're done, turn the original portrait right-side up. Get another sheet of paper and try to draw it again, this time, like the portrait, right-side up. Spend exactly as much time as you did before.
When you're done, compare the two drawings. Chances are, the first drawing turned out noticeably better. But why? Because when the portrait was upside down, and you were drawing upside down right along with it, it was harder for you to identify and categorize each part of the portrait, and thus your right hemisphere had an easier time taking over. When you drew the picture right-side up, your brain kept categorizing everything you were seeing. Suddenly, for example, you weren't drawing a series of simply curves--you were drawing a nose, and drawing a nose is hard. By avoiding your left brain's assumptions about how things should look, you were able to draw better.
This final exercise isn't a "trick;" the idea is simply to help you keep the right hemisphere of your brain active and improve your drawing. Grab a handful of large pieces of paper, preferably bigger than the typical 8.5 by 11-inch sheets. Focus on something--a person, a landscape, it doesn't matter--and sketch it using the entire sheet of paper. Set a timer beforehand for 20 to 30 minutes and don't check the time when you're sketching. The goal is to sketch as fast as you can, as fast as your eyes can see. Sketch the broad angles at first and slowly get more detailed as you go. Do not start over before your time is up.
When the time limit is up, put aside your piece and start an entirely new sketch, taking the same amount of time. Draw three or four sketches in this manner in a row and then compare them afterwards: go over whether you got better as you went along (unless you get tired easily, your first will often be your worst) and what you can improve next time.