Don't be afraid to get shot, it will happen. Get in and check people you are there to check people not to stand on the side lines and watch a game.
Don't stand behind a player, they won't get hit there you need to stand somewhere where you can see where they are likely to get hit.
Once a call has been made you can't change it, right or wrong. If you are wrong and a player challenges you, for example if you pulled them for bunker rub, just say there was paint there, that looked like a hit and you made a call.
If any player starts mouthing off because your an idiot, f*cked them over etc, if it's during a game tell them to get off the field or you'll pull another player, if they keep arguing do it, if they're clever they'll stop it and more importantly won't do it again on your field.
If it's after the game you can just remind them that they'll have another game or penalty point them if appropriate.
Be firm in the first few games, once players have been on your field or have watched games they'll know if they can get away with anything. Don't take any crap and people won't try to give you any.
People don't try half the stuff at Millenniums they do in local tournies.
Be firm but be polite, no one likes a marshall with a god complex, you just look like a d!ck.
Know the rules and make a call, i would rather people make the odd wrong call rather than just standing on the side line and not make any call.
Don't call players by their names even if you know them, it'll help avoid calls of bias.
clean the barricades between games, if there is no paint to rub against then there'll be less hard calls and less arguments afterwards.
So that's it, basically get in, make your call. Don't take any sh!t but be respectful and you'll get less sh!t and be treated with respect.
Get in and pull people out, don't give people a chance to play on, you'll have to pull less penalties, people playing on won't shoot anyone so there'll be less arguments.