Hi Stu,
Never had the pleasure of being best man myself but a fairly good opening line I heard at a wedding recently when the best man stood up was:
"This is the 2nd time today I've stood up from a warm seat with a piece of paper in my hand"
The line isn't that funny in itself but the reaction around the assembled crowd was quiete good as it took people a while to get it and when they did the young ones laughed at the simple toilet humour whereas the oldies gave the best man a look of disgust for lowering the tone!
Good luck mate