It has always been easier for me to remember shapes than numbers. For this reason, I have never thought of a course as fence 1, fence 2, fence 3, etc., but have always memorized the shapes of the lines within a course.
To do this, separate the segments of the course into outside lines, inside (diagonal) lines, half-turns, half-turns in reverse, circles, figure eights, and so on. In equitation and jumper classes, it is particularly helpful to think of the segments as shapes, since the courses are usually more complicated than those found in hunter classes.
Memorize your course as early as you can and be sure to walk it before you ride it. When walking between the fences, pay attention to your approaches to the lines and decide at what point you should look at the upcoming line. Work this strategy into your plan, so that your eyes will automatically turn toward the line when you get to certain points on course. Discuss the course with your coach; or, if he or she is not available, go over it several times in your mind, until you are sure you know where to go. Finally, think about your plan just before you enter the ring, so that it is fresh in your mind.