Banding the mane is a big one, looking good yourself, smiling, walking and standing straight, presenting the horse like you are proud to own the horse all make an awesome picture. I honestly have no idea about the whole judge and you placement thing, as I've never done a showmanship class, though I do want to do a halter class with my Arabian some day. I also agree about the halter being too big for the horse which doesn't present a very good picture. I would definitely practice at home as well, make sure that if they ask you to trot, that as soon as you ask for the trot, and start going yourself, that your horse follows instantly, no 3 steps of walk then an "okay maybe I'll trot". Make sure that she is right at your shoulder, not behind you or in front of you as well when you are moving. Good luck, and definitely keep trying, the more practice you get both at home, and at a show, the better you will both become.