Many moons ago, back when dinosaurs roamed the earth, I rode in a H/J clinic in Bend Oregon with Olympian Kevin Freeman. I learned a ton in that clinic, but one particular thing has really stuck with me for 20+ years. He talked a lot about habitual posture and about how you move, hold yourself, and sit in everyday life will be reflected in your riding. If you slouch driving the car, you will slouch riding, if you have low muscle tone walking, you will have low muscle tone riding, and etc. for whatever postural flaws you exhibit day to day. He recommended fixing these postural flaws when NOT riding, so correct equitation was easier to achieve when riding.
The true kicker of this quick talk though, was that in the saddle the human brain is only capable of working on about three corrections at once. So, say you ride chin forward, hands flat, and soft core. First, for a few days, you would practice keeping your chin to your collar and having a flexed core when walking, talking, driving and etc.. Then, in your rides you would keep this checklist running through your head until one item is habitual and then you would replace it with the next priority in your equitation. Over the years, I have found the three corrections rule to be totally true. Having more than three items on my "correction" checklist led to chaos but mentally I can keep up with about three things and this has been a great way to improve and solidify my position, whether it be for jumping or dressage.
So, this is a long winded explanation of the why of the checklist. It has changed many, many times over the years but has been such a valuable tool. My current checklist is 1) heavy elbows 2) project the whole front of the body forward and open 3) pretend I am holding a squishy tennis ball behind each knee. Nothing stunning to any dressage rider but super helpful for me personally, to make my aids more effective.
I have found the key is with the choosing of your three things. When you pick the three thing carefully, it has a sort of exponential effect. Take for instance the heavy elbows. This fixes(finally!) my vertical alignment(I CAN sit up!), connects the horse's mouth to my core, and makes me ride from my core/center instead of from my hands. Kind of a lot of fixes for one simple correction.
What's on your riding checklist and why?