Heather Moffett's site, who teaches you how to absorb your horses movement correctly so that it causes no damage to the horses back. She does this via her simulators. You can find her saddles on there to and her own forum, which is full of lots of info and friends. She also trains her own teachers to go out and teach others to ride correctly for the sake of the horse. You can find Becky on there to who does all the ground work with her horses who also does clicker training.
When we are on the horse I am a great believer that it is us with our positions that block the horses movement, if we are tight in our bodies tense somewhere. When I am in the correct position my gal will carry herself alot better and so do other horses. That is why if we are teaching our horses to be collected and carry themselves correctly we should only do the same with ourselves.
So this is why I do pilates to free up my body and become strong in the right areas allowing me to be stable and aligned.
And that is why if I was a beginner I would send them on one of Heather's simulator courses as it is amazing what beginners can do after being taught correctly how to absorb the movements of the horse. Many on EE send there other halfs to go on the course and they come back ending up being able to ride better than them in a few days when those who sent them have been riding for years and struggle to do the same
as we have all the past habits to release of that we hold.