I was best man twice for the same guy. First time I did the standard speech which was fine. Second time was a bit tricky as half the guests had been at the first wedding.
I started with exactly the same speech (I still had it from the first wedding 10 years earlier) and after the first 5 lines stopped and said "I guess half of you know this already!" then walked out of the room with a dejected look on my face. I could hear the murmuring as I left.
I grabbed my guitar and went back in and explained that rather than taking the opportunity to take the mick again I'd rather let my best mate know how I feel about him and sang May you never by John Martyn. Result rapturous applause all round. Groom in tears, his mum in tears, his new wife in tears in fact almost everyone in tears and I got off with a really hot bridesmaid that night.
Job done.
[youtube]http://www.youtube.com/watch?v=LOi_wxypeGc[/youtube]