It may sound overly simple, but a dressage test is truly a test of geometry. Judges will score an average mover that makes accurate figures higher than a fancy mover that drifts off the rail and makes egg-shaped circles. Knowing this, you can pick up a lot of points just by being the most accurate rider at the show.
Download these diagrams of the two sizes of dressage competition arenas to help visualize how your test should look.