From my experience,the most important thing to remember when marshalling at any rally other than staying safe, is to approach the spectators in a polite and friendly manner. If a group come towards you, approach them and tell them what time the cars are due, which way they are coming and what time they are due back if it is run twice. then tell them where they're not allowed stand. don't ever roar and shout at anyone. Just walk over to them and ask them would they mind moving some where safer. if an individual won't move or if a couple of spectators arrive late and try to stand in front of the people that have been there a while, try and get the crowd on your side to move them by saying these people have been here for the past hour waiting, and if that fails tell them that you will stand in front of them and block their view.
Show the spectators the safety plan for your junction and always make sure you and your fellow marshals allow people to stand in the same area's.
One other point, some of the spectator control's get a bit excited and carried away on the day and can undo your good work with one sentence over the P.A.
Always ask them are they happy with the way your junction is marshalled. If there is a problem walk over to the spectators and ask them to move.
Best of luck to all who go marshalling.