Having worked with my pony today, I was reminded of something my instructor taught me about the riding aids...
The riding aids are meant to be applied softly and accurately (in the right place, so to speak) in order for them to have the desired affect. If a horse can feel and respond to a fly sitting on its back then it is more than capable of feeling the aids, even the most subtle ones. In modern riding today, however, many riders use the aids too harshly, resulting in the horse becoming numb to the aids. They then either sell the horse (because it is unresponsive/lazy) or they start to use spurs and whips.
There is nothing I hate more than to watch dressage riders flapping their legs against their horse's sides. With a classical seat the legs shouldn't be moving around at all. They are supposed to lie still against the horse's side, wrapped around the sides like a wet cloth. Only the thighs, knees and upper calves should be in contact with the horse. The rest of the leg just lies where they naturally fall. When the rider should choose to apply the leg aid, he should just gently squeeze with the parts of the leg that is in contact with the horse. The heel should not be used as the leg aid. The heel is only meant to absorb the rider's weight, helping him to be balanced on the horse. It is only with more collected work that the heels might be used in the leg aid. Yet, I see so many riders (even top dressage riders) pulling their heels up to apply leg aids. And what a ghastly sight it is!!!
The hands should be used softly when rein aids are applied. Pulling a horse in the mouth does more harm than good. When riding turns and circles, pulling the reins will most likely have the horse fall in on the inside shoulder. When wanting to apply rein aids you should just have to squeeze the rein like sponge to get the desired affect. But this squeezing mustn't be a continuous pressure... my instructor taught me that I should squeeze and let go, squeeze and let go until the horse does what was asked. If you squeeze and hold you end up turning the squeeze into a pull. Horses by nature push into pressure. So if you just pull on the reins, the horse will just ignore it. And it hurts the horse because you are pulling against their sensitive gums. I had this with my pony. If I just pull my left rein she would just pull against it and try to move to the right. You could be pulling the bit right through her mouth and she'd still fight the pulling. But when I just squeezed my reins a little bit at a time (supporting with my legs aids) she would move in the desired direction. My instructor explained that squeezing works, because there is nothing for the horse to lean against. And it is as if you are asking the horse to do something instead of demanding it. If you had to chose, wouldn't you rather have someone ask you to do something than yell at you to?
Ultimately, aids are meant to be invisible. This is what every GOOD rider should aim for. It takes time and patience to learn this, but there is no better lesson (regarding riding) to learn than this. How we use our aids forms the basis of our riding. Without the aids we can't hope to communicate to the horse what we want it to do. It is a language that should be understood and mastered properly by all riders.
I found Sylvia Loch's 'The Classical Seat' a very good read. She explains all aspects of the seat as well as the aids of riding. A must read for any rider wanting to perfect their riding skills.