Pre-Parade, drink sufficient amount of water to keep yourself hydrated, not too much till bloated though.
During parade, just be relaxed and not too tensed. Breath in large yet constant amount of air. Wiggle your toes. You should do fine.
But remember to follow your normal rules like look up, stomach in chest out etc.
During our school's Speech Day this year, there was actually this NCC GOH cadet that puked onto the officer behind the Guest-of-Honour during the inspection of the GOH. He drank too much water beforehand and was too nervous during parade.