Riding into your corners will not only gain you extra accuracy marks in your dressage tests but will also give you more time and space to balance and correctly prepare for the following movements.
In a school where most riders cut their corners a path is quickly cut into the surface as you can see in the photo and it is far too easy to follow that track rather than riding into the corners.
A very simple tip to make sure you ride your corners is to put an obstacle in the corner which you have to go round. Here I have placed a blue barrel in the corner which does the job perfectly but a cone, chair, jump wing or other similar thing will also to the job.
I have placed the barrel a decent distance from the corner here which even a unbalanced youngster ridden correctly should be able negotiate which really illustrates how much most people cut their corners as the barrel is inside the standard track which is actually very similar to the track ridden for a 20m circle at the end of the arena.
As you improve you can move the object closer to the corner . I have seen photos of an instructor sitting in a chair only a small distance from the corner and asking their pupil to go between them and the side of the arena. Although that was taking riding into corners to an extreme it meant when the chair was removed the pupil still rode correctly into the corner.