If you will be working closely with the horse or doing portrait type shots, a pocketful of treats will prove very handy. The horse will be more comfortable around you, and more intrigued by that tasty smell coming from your direction. They’ll look at you more, which is great for portraits, and might even follow you around if you start to walk away.
Keep facial features in mind. Perked ears mean a horse is interested, while ears pinned back against their head indicate anger or frustration. The eyes are the most emotional part of the photograph. If they are not in focus, the picture is near meaningless. Watch carefully, however; if you can see the whites of their eyes, they are frightened.
Use a high shutter speed and low ISO when shooting. This is especially true when shooting outside, or at outdoor shows. If you must shoot inside, you may have to sacrifice the low ISO to keep the shutter speed. 1/500 sec is usually the lowest preferred shutter speed for action shots, with a wide aperture around f/2.8. On a bright, sunny day, an example of a possible array of settings might be: ISO 100, shutter speed 1/840, aperture f/4.5.
O yes and don’t get kicked in the face!