First, pay attention to your turnout. You don't have to be a slave to horse show fashion trends, but you can make every effort to be clean, tidy and wear tailored clothes. A sharp turnout tells the judge that you're serious about competing. There is simply no excuse to enter the show arena with dusty boots, for example. It takes but a moment to knock off the dirt with a rub rag.
Hairnets keep a rider looking tidy.
That means no ponytails or braids once you're past the small pony division. And a neat fitting hunt coat reveals a rider's posture. No one can look tall and secure in a baggy jacket. Second, learn to show yourself off to your best advantage. And know when to take cover and fade into the background when you're not doing well.