h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I once used the line
The nice thing about being Best Man is that you never have to prove it

The important thing to remember is not to make unguarded comments about any afflictions suffered by family members.
For example; try to avoid references to the fact that the Bride has just had her wooden leg creosoted in order to avoid rotting whilst on honeymoon, waterskiing. Or that her Uncle - the Dentist is like a Rhino - thick skinned, and charges a lot!
MOST IMPORTANT is to not mention the brief affair that the Bride had with "Uncle" Barry, that left her pregnant six weeks before the wedding.
You get the general idea.

My brother dropped his brides ring during the ceremony he dropped the ring. He was a student rugby player and I was worker in a jewellers. This was just after Rugby Union allowed professional players my opening line.

"I see my brother chose wisely in picking a professional jeweller to be his best man, I just didn't realise he would need a professional rugby palyer to catch it."

Some people didn't quite catch this until they either asked me or saw the video

My friend used this immortal opener:
'This is the second time today I have stood up from a warm seat with a piece of paper in my hand...'
which broke the ice and relaxed everyone in the room. His speech was one of the funniest I had ever heard, especially when he got heckled by both the Bride and Groom

I was joint best man for his wedding a year or so back and my co-best man and I cooked up a joint speech, with the pair of us doing sections, but supporting each other with comments flowing back and forth. Everyone there said it was a great speech and that the comedy was hilarious. I opened with the line 'The Groom wanted a best man but couldn't find one, so he got two slightly good ones instead' it brought the house down

There are plenty of websites now that provide help and information on writing speeches like these and there are also sites that will provide you with basic 'speeches' for free and for a small fee wll produce a custom made speech for you, if you aren't very good at writing things like that, based on information you supply.