Since I've only had formal training in the last year or so, there were many things that I never learned until last year. Posting to the outside diagonal was one of them. Man, oh, man that took me forever to learn! I am not 100% accurate, but I am getting much better.
Back to the expert's answer ... (by the way the column I am referring to is on page 72 if you get the magazine.) Elizabeth Madlener, USEF "S" judge, suggested that the rider check her "position and be sure that you have your weight on the inside seat bone." She went on to say that "if your weight is to the inside, your horse will throw you quite naturally up with the inside hind." And of course, "if you find yourself consistently coming up on the outside diagonal [outside hind?], then you need to get your weight down into the inside seat bone and leg."
Well woohoo for me! I wasn't sure how I was doing it, but I nearly always get the correct diagonal when we return to rising trot from the canter. This must mean my seat and leg are in a somewhat correct position!
Not one to look a gift horse in the mouth, I'll take the atta girl!