Adding a bit to the flood of good advice about discipline:
If it helps, think about the progression in pressure that you use to teach something to a horse for the first time (disengage the hindquarters, for example, since it is a fairly easy movement for a horse to get)
You have a young horse who has never had to move away from pressure before. You begin by focusing your energy on the hindquarters (that "stalking" or "predator" stance that was mentioned earlier in the thread) that is level one of pressure - the least you can possibly put on the horse. If the horse does not respond (which they predictably won't at first) you move towards their back end, maintaining that energy (level two pressure). Level three is putting your hand out towards the horse, level four is swinging a rope slowly, level five is swinging it quickly, six is smacking said rope on the ground or cracking it, etc.
If your horse is still standing dead still when you are wildly swinging a rope around in the air you HAVE to continue to up the pressure, this means *gasp* making contact between rope/whip/whatever and horse. You increase this pressure until the horse moves and then you stop immediately. Horses are good at making connections between "the human stared at my butt, then things got more uncomfortable from there, then I moved and everything was comfy again" eventually all you have to do is give a hard look at your horse's back end and they will move it away from you.
On a quick side note - it is also important to teach your horse when you are NOT asking them to move away. With mine, a firm "woah" will stop her from moving if I approach her from behind and want her to stand still, and if I am touching her (passively) she stays put. Otherwise her head is facing me all the time. Failure to do this could result in a horse whose back end you can't get to, as they disengage it whenever you try and move towards it (to pick up feet or adjust a saddle, for example)
Now - that is training. If our hypothetical horse had moved it's hindquarters TOWARDs you when you start applying pressure, that pressure gets upped to level X pretty darn quick as they are doing something disrespectful/the opposite of what you want.
The important thing to remember here is that all horses are individuals with different thresholds for pressure from people. Go from level one to level six with the wrong horse and they will bolt (or kick if they feel trapped) using less pressure than you need will be ineffective and result in bad communication between you and the horse.
To apply this idea to another example, let's look at a horse that rubs its head on its owner. (yes, its very adorable and your horsey loves you but you are not a wooden post, darn it!)
Lots of horses check in with you by wiggling lips on your arm or resting their chin near you. I am fine with this as long as it does not effect my posture or make me move.
My response, however, to a horse lightly pushing my arm with its nose is going to be a LOT different that one who rams my shoulder with its whole head. The first one is going to get a quiet elbow shake, while the other is going to be faced with me waving my arms and backing them up. Again, however, it depends on the horse.
It is probably better to overdo a correction that be too soft on your horse - but you want to make sure you're not completely loosing all training value by frightening your horse and loosing their focus.
This is where desensitization comes in. I can crack whips and swing ropes and leap around like a crazy person, but if I am not focusing my energy on my horse she acts as if nothing is happening. As soon as I do those things with my focus on the horse, she reacts.
Wow. That turned out WAY longer than I expected it too - hopefully it makes sense. NH is not about treating your horse like a delicate flower, its about treating them like a horse in a way they understand and respect.