Holme, listen to what I am about to say here because it will mean you then having to ignore some of what's been advised already.
It's all well and good 'not taking sh!t' from people but if you go in too hard with that authoritarian attitude; it can blow yup in your face especially coming from someone who hasn't done the job before.
Make sure you are firm AND polite, and try not to read too much into 'not taking sh!t' because it will set you off on the wrong foot believe me .. remember, these guys are paying customers and should be treated as such, and not like some naughty child.
"customer is always right" attitude
when i marshalled I generally followed that, and it works, unless you get some guy or girl ignore the "please keep your goggles on" warning for the 3rd time then thats when I got slightly annoyed and used the "If i have to give another warning you'll have to sit out the next game for your own safety" speech, depends entirely on who the group is though, normally find kids the worst, especially the trigger happy little so and so's who although good for business, are not good for your health haha
on a seperate question, what do you guys do when you get a forager e.g. someone who doesn't want to buy more balls, so just picks them up from the field, i normally say that if they do it and the gun breaks, they'll be liable
normally works haha, forgiving if they run out in a fire fight and it's the odd ball
cheers for reading