Exercise: Ride EVERY stride of a line straightly.
For example, instead of looking at C and riding for it down centerline, follow the imaginary line from A to X to C through your horses ears and keep them on that line every stride. At any moment, your horse should be exactly the same distance from both long sides.
Note: That you have to keep all of your aids ready all of the time. By paying attention to where you are, you should also be able to fix your horses wiggly much sooner... instead of getting back on track as you approach X, you will be staying on track the entire time. By looking through your horse's ears, you will also notice if he is staying steady in his frame.
Apply: Pick an EXACT distance to stay off the track and ride your warmup on that inside track. At any moment, you should be able to measure your hoof marks and find that they are within inches of that selected distance. Remember to follow the selected line through your horse's ears.
Continue to do the same on perfect 20-meter circles and diagonals. Remember to focus on the imaginary dotted line from each point, through your horse's ears.
Confirm that you know how to ride the points of the following:
20-meter circle in the center: Hit at E, 2 meters before I on the centerline, B, 2 meters before L on the centerline.
20-meter circle on the ends: Hit at C/A, 4 meters after M/H/K/F on the track, 2 meters past I/L on the centerline.
Diagonals: HXF, KXM