I bought my boy when he was 6 months old. He was a little devil and gave me no respect. It really just takes time and CONSISTENCY. He would pretty much try and walk all over me when he was young, pull on the lead, shove his shoulder into me, nip me on the butt alll the time. I had had plenty of experience with horses before him so I new the way to treat him in order for him to respect me. You have to be really strike when they are naughty (haha, I love that term), but also give tons of positive reinforcement when they are good. Horses are supposed to pick up on things 4 times faster then humans!! (I can see that it makes sense since they are prey animals and their lives depend on it).
So, pretty much what I did was read a lot and went to some demonstrations. I saw the Parelli demonstration and it was really educational. One thing that has helped me with the biting and respecting my space was something called "hide the hiney". Odd as it may seem, it does work. When the horse nips you or drags/pushes against you and you are leading him, you need to quickly turn your body away from them and smack them on the butt with the end of your lead. I forget the reasoning of why exactly this works but I am sure you can check it out on the internet somewhere. Smacking them with the leadrope sounds mean but I am not talking about thrashing them with it, just one straight up smack - then continue on as you were doing. WHen you put it into perspective, they are horses, BIG animals, so that one smack from you with the rope doesn't really hurt them, but more or less gets the point across. Also, if they nip, do not playfight with them, because that is what they are looking for and will just continue. AND, don't "get up in their face" area b/c that could lead to a horse being head shy.
I most likey babbled but I hope you got something out of there.