The German magazine "Dressur Studien" (means dressage analysis) has published and shared a poster that compares horses in right and wrong posture when ridden. I think it's a really important issue:
One of my favorite topics is preparation. Maybe because I often find myself neglecting it and having to face the consequences. Like lately. 
I wanted my horse to do haunches-in like in the picture below with me guiding his hind end via rope to the inside. It's quite a sophisticated way to do it - I saw Parelli-Instructor Walter Gegenschatz showing it during a demo and was quite impressed. 
My horse knows how to do haunches-in when tipped on his back from above. But as he yields to pressure I thought he would be able to figure it out with the rope around his butt too. I was wrong. Instead of a nice and clean haunches-in I got a horse pushing into pressure with his hind end and struggling to get his head free (as the rope that leads the haunches is attached to the nose too).

KruppehereinWell, I started all over again and checked what I missed in the first place: how good was his yield to my fingertips on his flank - nonexistent. I reminded him that pressure there still meant "yield please" and he was fine with it within a few minutes.
But no wonder he had trouble with the rope when he obviously wasn't able to yield to a feel that he already knew. And there comes the right preparation into play. Even if we have asked a horse a thousand times to do a certain maneuver it doesn't mean he will always execute it in the same quality. So it is a good thing to always start with the easy thing first and build up to the more difficult thing - not the other way round like I did it. It sounds like common sense, but unfortunately there is a difference between common sense and common practice.

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Even if you don't like what I have to say.
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Thank you!


PS: Send me your questions. I will pick one of you who commented, answer your question and try to help you.
When it comes to solving problems, the human will most likely choose a way that does not help the horse. Let me give you an example: The horse will not stand still while mounting. The typical reaction of a human would be to have someone else hold the horse to stop him from moving his feet.
That might fix the problem for a little while but the reason - why the horse moves in the first place - is not addressed. Most likely the horse will try to evade the mounting situation in another way. He might pull back, shoot backward or refuse to even go near the mounting block. 
Instead of holding him, it would make sense to think about why the horse moves when mounted. He can have experienced trouble when a clumpsy rider got on and disturbed his balance. But more likely this issue is not about getting on but about a lack of preparation. The horse might not have learned to stand still and accept a human at his side, cinching, adjusting the stirrups - and mounting. 
This concept is emphasized in the videos of Warwick Schiller. Be it the horse standing still, giving his feet or being led away from his best buddy: Schiller usually doesn't address the problem at hand but his relationship with the horse. He established a respect system from the ground and makes sure the preparation is solid. And the problems disappear one after another. 
So make sure your horse is prepared and to take it step by step - without skipping steps!