I'm going to answer for my daughter b/c it has been a loooong time since I have had an instructor and that was as a beginning older rider. When Steph moved to St Louis about 10 yrs ago it took her awhile to get back into horses. When she did she didn't have her own horse and sort of wandered through several barns due to safety issues, cliquism (new word), not the high quality of training she was looking for, less than adequate facilities and so on and so forth.
She found her trainer by asking the instructor she highly respected at a riding clinic, who gave her the name of someone she knew in St Louis who was very happy with her trainer. They clicked and have been together since.
Alix is demanding, but very patient, very outgoing, laid back (this IS the eventing world we're talking about), knowledgeable about how to fix things with horse and rider and the communication between the two.
Finding the right trainer is a very personal decision. What works for one won't for another, but asking around at clinics and other riders is the way to go. And I second the value of private lessons. In most cases you don't pay too much more and the personal attention is so worth the extra $$$.
I am not one of those who in expressing opinions confines themselves to facts. - Mark Twain
The fact that man knows right from wrong proves his intellectual superiority to other creatures; but the fact that he can do wrong proves his moral inferiority to any creature that cannot. - Mark Twain