When I watch others ride during lessons, I often find students do not pay enough attention to executing all the transitions well. This can be from gait to gait or simply linking one exercise to the next, i.e. transitioning from walk to trot or going through the corner and turning down the centre line. The moments in-between are crucial to each exercise.
For example, you are going through a corner followed by a half-pass on the diagonal finishing on the opposite corner. The whole exercise is comprised of turning from the corner, executing the half-pass, straightening and following through the final corner. If you do not turn properly, your half-pass will not begin properly. If you do not complete properly, you have begun your final corner improperly.
I find it helpful to think of the beginning, middle, and end of everything I do. If I'm doing the exercise mentioned in the above example I think:
1. Complete the corner and turn the horse onto the diagonal. Once straight...
2. Position the horse for the half-pass.
3. Arrive at the track just before the corner, straighten the horse's body until all four legs are centred on the track, and enter the corner.
Remember there's a whole half horse's body behind you where you cannot see so you must feel. Connect the dots between the movements. No step should be taken that has not been accounted for. This will help you have a smoother, more even ride.
A good refresher is to look at the directive ideas on the national or FEI dressage tests to know what you should be looking for. These are available on the Equine Canada and FEI websites.