A. "Start by dividing the whole space unequally with a single line. It is best to avoid placing the line at a point which would be one-half, one-third, or one-fourth of the whole space.
B. "Then draw one diagonal of the whole space from diagonally opposite corners.
C. "At the intersection of the diagonal and your first line, draw a horizontal line across the space.
D. "Now draw diagonals in any of the resulting rectangles, but only one to a space. Two diagonals crossing like an X would divide the rectangle equally, which we do not want.
E. "Now draw horizontals or perpendiculars at any intersection, thus making more rectangles to divide by diagonals again. In this manner you will never break up the same shape twice in the same way. It offers a great deal of suggestion for the placement of figures, spacing, and contours, with no two spaces being exactly equal or duplicated, except the two halves on each side of the single diagonal. If you have a subject in mind, you will begin to see it develop."
Here are a few thumbnail sketches I did using this technique. In the first and third I used the same photo reference (notice the tree on the left) to show just how varying the design can be.
Thanks to Andrew Loomis Creative Illustration.