1) Brush him up and make him look good - a horse with mud caked all over is still a horse with mud caked all over.
2) Choose a good time of day, when the sun is lower to the horizon. This will create more dramatic photos, with very rich color (see my examples in "Maia 8/28/08")
3) Again with lighting, have the sun behind the photographer, and make sure the photographer's shadow isn't in the picture, or you can end up with this: http://i237.photobucket.com/albums/f...ugust28029.jpg
4) Put on nice tack, and Showsheen is a good idea.
5) Have at least one other person helping. One to hold the horse, and "get ears" and the other to take the pictures. ("Getting ears" means to get the horse to prick his ears forwards)
6) On the subject of "getting ears" here are some tips: a) Use a plastic bag on a whip, and shake it low to the ground. b) get a paper plate and put some gravel in it, and shake it low to the ground. c) jump around like a madman and yell things... horse will usually prick its ears then ignore you, so this is only good once, maybe twice...
7) With a digital camera, you can depress the shutter button halfway to make it focus on various subjects. Depress the button halfway, then wait till the horse "has ears" and is looking pretty, then fully depress it (don't let it come back up before this though, just depress the button the other half of the way!)
8) Angles are important. Make sure you are perpendicular to the horse's body if you're taking a shot from the side. Get level with the horse - you can exaggerate conformation faults with the wrong angle.
Bad angle: http://i237.photobucket.com/albums/f...ugust28017.jpg
makes Maia's booty look huge (it is, but no need to make it look bigger!) and her shoulder look bad. Her neck looks like it's attached weird as well.
Better angle: http://i237.photobucket.com/albums/f...gust280182.jpg
9) Legs - get the horse standing with all 4 legs visable; front nearest you ahead of the front on the other side, and hind nearest you further back than hind on the other side. This creates a good illusion with the legs compared to the body. "Hiding" one leg can make the horse appear unbalanced.
10) Hoof polish goes a long way!
Okay I think I've rambled enough!! Good luck, can't wait to see the results!
Edited to add... I have more pointers for action pictures, but that's a whole new can o'worms... let me know if you'd like those as well!
There is a great book called "Photographing Horses" that is an amazing read!