"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