Learn to play an instrument. Ones you can walk with are the best to use for marching bands, such as clarinet, flute, trumpet, saxophone, or even a base drum.
Practice, practice and practice some more. For marching band competitions, bands usually play three or four songs. Judges not only judge on how the band marches but also how well the band plays while marching.
Master the steps for each show. Always be a little bit ahead of yourself and know where you need to be on the field. You are part of a whole and one little mistake can and will be noticed.
Position your body correctly while marching and playing. Learn proper bodily alignment depending on which instrument you play. Posture is important when marching because it helps you to keep your balance.
Buy proper marching shoes. Some marching bands use black shoes while others use white. White is the hardest color to march in because if you make a mistake it will be easier to see opposed to if you were wearing black. However, marching shoes regardless of color will help you maneuver around the field as well as keep proper marching technique.