Your tack and equipment should be well cleaned and oiled. I prefer to use a leather halter over a nylon halter. Your saddle should be cleaned and your pads white or washed. Bringing along a rag to do last minute dust offs is a good idea. Your Equipment Equipment is more than just a camera with film.
Lighting is everything when it comes to a good photo, as well as, background.
Really look at your surroundings and be picky when it comes to posing your horse. Sticking to a simple background will make your subject, your horse, stand out. A grassy setting can really show off your horse if it is not too high. Make sure the lawn is trim so that the hooves are not hidden in blades of grass. As for sale ads, standing him up on a clean flat surface like a driveway can do the job.