Is this your first time doing Showmanship?
Showmanship is by far my favorite class to show in and judge. It's all about precision and polish. The judge will be looking for turnout (you & your horse! Both should be spotless and well groomed, halter (or bridle if going english) cleaned & polished, all ends tucked into keepers) Quality of your pattern, precise & crisp. Nailing your switches and doing them efficiently, I like a 3 step switch myself - start with your outside leg first, outside cross, inside cross, outside to bring your feet together - very fluid and efficient once you get the feel. Be sure your horse is set up perfectly square and stands still. NEVER touch your horse in a Showmanship class and NEVER touch the chain on your lead shank. Elbows close to your sides, especially when trotting with your horse, if they aren't close and flop around it looks like chicken wings
Little things can make the difference between 1 & 2. Things like looking back over your shoulder when trotting away from the judge. Being prepared & set up before being called to do your pattern - nothing is more frustrating as a judge than watching someone fidget with their horse to get them set when you've been acknowledging them to start their pattern and they aren't paying attention. Get your horse ready and be ready before it's your turn to work. A quick nod when acknowledged to work your pattern as a courtesy is nice. If you get a judge that asks questions, if you don't know the answer be honest "I'm sorry I don't know the answer to that m'aam (or sir) but I will find out for future knowledge." If you do know the answer be as technical and detailed as possible, ie "Where is the poll" - "on top of it's head" isn't as impressive as "It is located on the top of the head, between the ears and is also known as the atlas/axis." Doesn't have to be a speech by any means but clear & concise with clear knowledge of the answer is good.
Practice patterns at home. Your age/division will determine how tough your pattern will be. At the youngest age, I expect walk, trot, back, a 90 degree turn & inspection. After that, it's almost a guarantee you will get a 270 or 360, multiple direction/speed changes (won't be just a straight a to b to c pattern in a line), back & inspection. The youngest I will line up side to side, older I always line up head to tail, for some reason it is a bit harder to get all of your changes right when head to tail. Work your horse and at least 2 on either side (or front/back) of you. I like to see a competitor work the entire class, regardless of where I am but don't ding for not doing so farther than 2 horses out. When you go in check out the color of the judge's boots & hat (this will help you see where the judge is when not right next to you)
Best advice to any showman "Sell your horse to that judge without saying a word." but the most important thing is to have fun! Hope that helps!