Novice riders tend to perch forward so my advice is to sit back and keep your head up, and do not mess with your horse's head throughout the class. It's called "checking and fixing" and will definitely affect your placings. Try to use your hips, legs, and spurs to guide your transitions and make everything look effortless.
Like others here have said, lots and lots of practice and transitions, but keeping the gaits clean and correct. Natural ability of your horse definitely helps as well.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela