What skills are needed to become a rally marshal?

I'm asking club members this, especially members who are or have been rally marshals. I have enjoyed rallies by my local centre and other centres I have attended and would like to find out more about being a rally marshal. The only member here I know who have done this job in the past is Firedragon. There must be more who read CT so please, put fingers to the keyboard and give me some advice. Many thanks in advance 

Bob