In Showmanship classes, the horse is basically a prop so that the judge can see how you have groomed, conditioned, and handle him. Halter classes judge the horse and his correctness of conformation and movement according to breed or type.
I show English Grooming and Showmanship. My horses have long manes due to their pasture dwelling day life, so I do a running french braid for their manes. I like to french braid their tails as well, unless the show is tiny. There is a book called Grooming to Win that has excellent braiding explanations and diagrams, that's how I taught myself to braid. If your horse's mane is pulled short, flat braids or button braids are in order. Sew them in if possible, judges notice rubber band shortcuts, and rubber pulls and breaks hairs. Use a commercial hoof polish, clear for white horn hooves, black for dark. If your horse has socks on some legs, but not all, experiment and find what looks best. Black against white hair can make hooves look bigger, and you should do all of the hooves the same color. Check your rulebook, if possible, but I would clip all the long hairs from your horse's face. This kills me, so I only do it for bigger shows around their eyes, but the muzzle is a must. Clip off the feathering on the lower legs and the billy-goat beard from the lower jaw. Pinch the ears shut and clip away what sticks out to clean up the ears. A little baby oil on the face right before the class adds a finishing shine.
Showmanship generally uses a crossover technique to follow the judge as he/she inspects the horse. You can find diagrams and better descriptions than I can give online.
Sorry that got so long, I hope it helped!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown