I don't have any advice for the horse, but I have advice for the clothing! I love love love fashion!
First, DON'T wear a busy pattern. Ex. Hobby Horse Clothing Co. - Verona Blouse LTD
Something with black paneling on the sides will slim you Ex. Hobby Horse Clothing Co. - Slinkies
The "black slims" is right, but the truth of that is, if you were one color from top to bottom it will slim you. The less busy your outfit looks, the less the eye jumps around, the slimmer you look. Dressing in one color makes you resemble a column. Imo, black is boring though, so if you do decide to go the all black route, I should suggest having something on you that is an interesting detail. ex. Hobby Horse Clothing Co. - Ritz SwitchIt Jacket LTD
When you dress, think of how the lines your clothes form draw the eyes. Vertical lines, be it the pattern in the clothing, or just the way the clothes draps will think you. Horizontal lines will make you look wider, as will dressing like a stacked blocks: one color on the legs, and a completely different color on top.
I do agree with getting spanx or some sort of body supporting underclothes. Those things are incredible! The makeup I don't agree with. Besides the fact that unless you really know how to put on makeup you'll end up looking really weird, you will probably look best wearing natural makeup. Ex. http://img264.imageshack.us/img264/6123/199mf0.jpg
Please no layer of eyeliner! You aren't a raccoon!
I don't know your skin tone though, so I wouldn't be able to recommend anything past that.
My personal thoughts on color- half the people in the ring will be wearing black. A quarter will be wearing sand or beige. The rest will be wearing some really bright gaudy color. If you are set on black, also consider hunter green, navy blue, or a deep red. They are the same tonal range, so you would get the same effect as black, but they will stand out. I also don't show western, and I haven't been to a western show in like 5 years, so take what I say with a grain of salt :)