Here are some guidelines:
(1) The sketch is small-postcard size.
(2) Sketch does not extend into any of the corners.
(3) The design should touch the sides at least once but need not touch all 4 sides.
(4) The composition is the cruciform.
What is a Cruciform? A cruciform is created when dominate vertical and horizontal shapes intersect. Where these shapes converge it is an ideal place for establishing your center of interest. The intersection is off center both vertically and horizontally. Care should be taken not to have the intersection in the dead center of the paper. The corners are simplified creating negative space.
A few quick sketches to show vertical and horizontal bands of a design using the cruciform.
Next time you sketch trying using the idea of a "vignette" to simplify.