Marshalling races is a fantastic way of giving something back to the sport.
I always shout and cheer and clap as much as possible because i know how much that helps me when I am racing.
It is rewarding in it's own right but it is great when the runners show their appreciation by say "thanks", waving or just generally wheezing in your direction.
When I am king it will be the law that ALL runners should marshall at least one race a year or their UKA card be revoked