Few things I've picked up
1) Avoid short focal length lenses and close up shots - you want to be back using a telephoto otherwise perspective distortion kicks in a lot with a horse and that closer to you belly suddenly gets a whole lot larger (horse ends up looking over weight - to say nothing of the depth of field issues). So shoot further back with more distance and a nice telephoto. 50mm+
2) If you are using any flash/softbox setup setup the lights and bring the horse to the lights not the other way around - that lets them approach slow at their pace to get used to them. Note that many horses today are more used to flash (heck most are likely being flashed seconds from birth with phone/point and shoot cameras) but that doesn't mean they are all spook free - so always check with the owner and work in a safe location.
3) Try to pick up a few body language hints - things like ears folded back and foreleg stamping - as signs of a stressed/upset horse. The trainer/owner/rider should be with you to spot these, but being aware of them yourself will help a lot. A general awareness of horse body language helps also for getting good shots - things like the already mentioned, ears forward (although a sound is good but a bag I'd avoid - seems any bag that blows and jumps around in the air can spook many horses from what I've heard).
4) Horses are big so don't underestimate how far back you have to be for a full profile shot - similarly keep a much bigger awareness of surroundings so that you can get some clean shots without too much distraction in the background (or at least control that background so its suitable).
5) Take some suitable treats (ask the owner before -but apples and mints are favoured options) for the horse. Introduce yourself and get to know the horse a little. Again its building a familiarity with the subject and should help the horse to relax more and pose more naturally.
6) Take it slow - have fun - expect re-shoots - its your first time with a subject you can't totally control. Shooting anything new introduces new skills to learn that you won't pick up first time; but if you take it slow and ensure those involved are having a good time it should go well.
As for idea check out:
https://www.flickr.com/photos/raphaelmacek/