... most of this dressage stuff is plainly too complicated and depends so much on each horse and rider that without standing in the middle of the arena, it is impossible to peg what will work and what will not.
And this is why I don't try to explain things here. The danger of misinterpretation is far too great for my conscience. There are many authors who can and have do a far more thorough job than I - Podhadjsky, Watjen, Swift, Wanless, De Kunffy, Savoie, and many more.
If someone is lucky enough to board in an area with a choice of trainers, then first for me would be RESULTS. Not just show results, but what I can see with my eyes - are the horses forward, sound, stretching to the bit, happy? Are the riders working constantly on bettering their seats - longe lessons, no-stirrup work? Are any questionable gadgets being used? Are the students happy with their progression?
It would be terrific for a Pony Club A or B rating, or a BHS certification. Certainly having the US rider medals are great achievements, also. But there are many very good trainers who for lack of funds haven't been able to show as much as they'd like, and these trainers should not be discounted if they are continuing their own education via lessons and clinics and producing good results with their students. This is especially true for riders just starting in dressage.
Example: A friend of mine recently switched over to dressage after 30+ yrs of H/J, and bought a nice 1 L horse who had scored very well in recognized shows. She began lessons with a well-regarded trainer who came to her barn once a week. After some months of lessons, she began riding with a new resident trainer, who lacked US show results but who was a Pony Club A grad.
The difference in my friend's learning curve is startlingly good! She has found a trainer who not only teaches dressage but teaches it from the seat up, with weekly longe lessons and stressing only correct technique on her own horse. My friend had hoped to debut at 1L this spring, but is now happily committed to concentrating on her seat and the connection, so 1L will wait until both horse and rider are ready.
She has found the trainer I'd hoped she would find; one I'd wish everyone would find when beginning dressage or committing to taking it seriously. We who have put in the miles know that it ain't about the tricks, its about the seat and the training scale. Finding a trainer who has these priorities is what is most important IMO.