The thing about being a Marshal, or any other customer serviced based job...is actually enjoying/caring about what you do, rather than just being there for the money. You can pay a 16 year old, £40 a day to do sod all and he will keep coming back, regardless of if he actually does his job well.
Safety is the other thing...you are responsible for looking out for idiots taking masks off, which means you have to jimp on their back is required, to keep their eyes in the right place.
As for punters not doing as they are told...well...that comes with an authoratitive manner...not, being arrogant and cocky...friendly, polite (as best you can!), but firm.
If they dont listen, and keep doing something dangerous...just inform them that, if they do get hurt, by not following teh rules, it will be down to their negligence...so dont expect a payout!
Also...threaten to end their days balling if they persist, but you have to make sure that you have the bosses backup..so best to point teh player out to him/her and let them have a word in their ear.