Preventing a horse from biting you is done by approaching them at the shoulder, paying attention to their mouths at all times and keeping them calm through touch and a calm voice. Avoid getting bit by a scared or curious horse with important information from a farm manager in this free video on caring for horses.
To prevent a horse from biting it kind of depends on why the horse is biting you. Sometimes older stallions it is just their habit to bite you. There is different reasons for horses to bite, some it is about habit and some of it is out of fear. In any case it is important to stand in the safe zone beside the horse's shoulder and always keep your face and your focus on the horse's mouth or nose. When approaching a horse that is prone to biting you approach them at the shoulder away from their mouth. If you do need to halter this horse or work on this horse, you then work your way up from the neck to the top of the head and down the face and slide your hand across the top of its nose to gain a little bit of control. I have been bitten by a horse a lot. Being bit by a horse can hurt a lot if they get a good one or it can just be a pinch. Very frequently the babies are curious and they tend to nip a little bit. Usually if a horse is going to bite you there is some kind of warning by pinning the ear so to prevent a horse from biting you always have your attention on the horse.
Specialty: Horse Health