It helps to be polite, but assertive (thinking along the lines of, you need a 18 stone rugby player, to do as he is told, and your a 15 year old kid!)..pay attention to what is going on and where, familiarise your self with the site guns (if recball marshalling), make sure you know the answer to any questions, such as where the deadbox is, game time, format and goal's, boundaries of fields etc.
Always carry a whistle and preferably a radio (walkie talkie) if the site uses them.
Do not be afraid to shout at people, for infringements, but explain to them afterwards why you shouted at them....learn not to become borred by shouting "KEEP YOUR GOGGLES ON PLAYER!!!!" every ten minutes.
Dont stand where you are likely to get shot...it's less fun when it happens all day, while your standing around.
If your marshalling adult Pikeys, just let them get on with it