Here are some of my favorite passages from Xenophon's book "Art of Horsemanship" written some 2500 years ago!  My comments are in red.

[5.9] Washing down of the legs we disapprove of; it does no good, and the hoofs are injured by being wetted every day. Excessive cleaning under the belly also should be diminished; for this worries the horse very much, and the cleaner these parts are, the more they collect under the belly things offensive to it;1 [5.10] and notwithstanding all the pains that may be taken with these parts, the horse is no sooner led out than he looks much the same as an unwashed animal. So these operations should be omitted; and as for the rubbing of the legs, it is enough to do it with the bare hands.

Sounds like horses are still horses and have always BEEN horses. They will get dirty no matter what you do!

[8.13] Now, whereas the gods have given to men the power of instructing one another in their duty by word of mouth, it is obvious that you can teach a horse nothing by word of mouth. If, however, you reward him when he behaves as you wish, and punish him when he is disobedient, he will best learn to do his duty. [8.14] This rule can be stated in few words, but is applies to the whole art of horsemanship. He will receive the bit, for example, more willingly if something good happens to him as soon as he takes it. He will also leap over and jump out of anything, and perform all his actions duly if he can expect a rest as soon as he has done what is required of him. 

Reward the behavior you want! A man after Skinner's own heart.

[9.3] Accordingly, at the moment of mounting, the rider should take care to worry him as little as possible; and when he is mounted, he should let him stand still longer than is otherwise usual, and then direct him to go by the most gentle aids.  [9.10] It should also be known that a horse can be taught to be calm by a chirp with the lips and to be roused by a cluck with the tongue. And if from the first you use with the cluck aids to calm him, and with the chirp aids to rouse him, the horse will learn to rouse himself at the chirp and to calm down at the cluck.

It is awe inspiring to me to see how old such tiny everyday things are!  What is more interesting though is it is clear that he understood that it is not the method per se but the consistent application that is important.  

[10.12] The mouth must neither be pulled so hard that he holds his nose in the air, nor so gently that he takes no notice. As soon as he raises his neck when you pull, give him the bit at once. Invariably, in fact, as we cannot too often repeat, you must humor you horse whenever he responds to your wishes.  

And Boucher thinks HE came up with raising the neck and release?  Everything old is REALLY OLD!!

[10.15] When your horse has progressed so far as to bear himself proudly when ridden, he has, of course, already been accustomed in the early exercises to break into a quicker pace after turning.1 Now if after he has learnt this you pull him up with the bit and at the same time give him one of the signs to go forward, then being held back by the bit and yet roused by the signal to go forward, he throws his chest out and lifts his legs from the ground impatiently, but not with a supple motion; for when horses feel uncomfortable, the action of their legs is not at all supple. [10.16] But if, when he is thus excited, you give him the bit, then, mistaking the looseness of the bit for a deliverance from restraint, he bounds forward for very joy with a proud bearing and supple legs, exultant, imitating exactly in every way the graces that he displays before horses.

Half halt, release, forward!

[11.5] We, however, consider that the lesson is most satisfactory if, as we have repeatedly said, the rider invariably allows him relaxation when he has done something according to his wishes. [11.6] For what a horse does under constraint, as Simon says, he does without understanding, and with no more grace than a dancer would show if he was whipped and goaded. Under such treatment horse and man alike will do much more that is ugly than graceful. No, a horse must make the most graceful and brilliant appearance in all respects of his own will with the help of aids. [11.7] Further, if you gallop him during a ride until he sweats freely, and as soon as he prances in fine style, quickly dismount and unbridle him, you may be sure that he will come willingly to the prance. 

I'm left totally in awe of this man and his obviously deep knowledge of the horse's nature. Kind of puts all the arguing about who is right or wrong, vis-a-vis German vs French vs whoever, into perspective.