This is a debate that could go on for hours because there is no simple answer. The first thing should be that it should be clear at marshall points which part of the track they are responsible for marshalling, and the marshall points should be positioned so that as far as possible they don't obstruct the view of the track. Then all you can ask is that Marshalls pay attention and don't watch the race for the lead or what their mate is doing (although we have probably all done it truth be known), but concentrate on thier part of the track. Try as best as possible to put cars on in the right order and try to avoid getting in the way of the other cars. Sometimes a driver will crash in a black spot on the track, or when another person is being marshalled, and that is just tough. Sometimes as a marshall you will make mistakes, but hold your had up and acknowledge it and most racers will accept that.
What gets my goat is:
1. Marshalls who think they are too good to be turning over cars in 'lesser' races and leisurely saunter over.
2. Racers who shout 'come on marshall!' (or worse) as soon as they themselves have made a mistake from pushing too hard or misjudging part of the track.
Great debate though!