Whenever you play, don't play so loud that your sound can be distinguished from all the other sounds on the field. A myth trumpet players especially believe is that if they play mega-loud, the whole band will sound loud. The truth of the matter is that if everyone plays at about the same volume, their band will sound louder. When you hear drum corps on T.V. play extremely loud, it is because everybody is playing loud, not because just one person is.
Don't jam the horn into your lips. This will greatly diminish your endurance and you might not be able to last even to the end of a show.
As far as how to get the most out of your horn so others can hear you, it helps to push out with your stomach as you go higher to maintain support for those notes. If you could get entire sections to do this, crowds go nuts because it sounds so great.
When playing a solo on the field, let the music blow all the way through you from your head to your toes. In order to get everyone to hear you, it sometimes takes a lot of air support and remember to push out with your stomach. That is easy to do on loud, fast solos, but on lyrical solos, you still need to play out. When playing these, don't play as obnoxious as before, but still maintain the support of blowing through your whole body.
Overall, try to march smoothly so you don't jostle your horn and damage your lips.