If a horse is trying to be mouthy or playful, and he keeps wanting to nip at you, boredom is also a factor. The best advice that I have been given is to just annoy the horse back. If he starts trying to get all up in your space, rub your hands over his eyes, all over his nose. Soon enough, he's gonna say 'hey, that's annoying, I don't want that,' and turn his head away from you.
A lot of people, wether they realize it or not, ag the horse on. If you always stand close to the horse's head so his muzzle can find your shirt, what do you think is going to happen when he gets bored? Horses are like babies, they always have to put things in their mouth that they smell and are interested in.
As for an aggressive horse, people ag that on too. John Lyons says the best thing to do with a biting horse is give it love. When a horse is dubbed as a 'biting' horse, of course no one wants to get bitten, but people will poke at it with sticks, yell at it, hit it in the face, so it only learns to be more aggressive. John says to just stand by the horse where it can't bite you and love on it, so it learns it doesn't have to bite you (he goes into a more detailed process, I will have to borrow that tape again) lol.
Lettin' the cat outta the bag is a whole lot easier than puttin' it back. -- Unkown