The most important thing is to prepare well. The fact that you're thinking about it now is a good thing.
The second most important thing to remember is that you'll be performing to a very receptive audience. People at weddings are wanting you to be funny and they will have had a few drinks by the time you speak so they will laugh at almost anything. They will almost be willing you to be funny so it is almost impossible to fail.
I was a best man 7 years ago and I based my speech on the groom's life story. I went and spoke with his Mum and sister and got lots of embarrassing stories of when he was growing up plus a great baby photo that I had enlarged of him crawling towards a pint of beer. Parent's are a goldmine of embarrassing stories. Plus by talking to his Mum I got her on my side so the chances of offending her were slim.
I then emailed all of his other mates asking for embarrassing stories about him. It also helped that his bride had emailed me asking me to make sure that he behaved himself on the stag night so I read that email out too. It's not traditional to embarrass the bride but she was a mate of mine too so I felt she was fair game and it went down very well with the audience! Plus it embarrassed the groom as it showed that already he was under the thumb.
Unless you are Peter Kay or Jimmy Carr I would avoid trying to tell jokes that you get off the internet or from best man books as it is too easy to get them wrong. Telling funny stories about the groom will work best as they are more personal.
Plus it doesn't hurt at the end to say something from the heart about the groom and how he is your best mate and has always been there for you etc. Get's the mum's and grandma's weeping.
And Uridium is right, make your speech 10 mins max as people will start to get bored if it's any longer.
And finally, good luck!
true, but then a few pre-speech drinks can change things on the day
Did the best man thing a couple of years ago, thought I'd put together a slide show of the groom and the happy couple over all the years I'd known them, including a few embarrassing pics from the stag do, and got other friends and relatives to email me pics as well, going back to infancy.
then put together the narration (my speech) based on a several running gags about the grooms beards, haircuts gone wrong, fondness for crisps, the bride's period as a crusty etc - did it all on powerpoint and printed off the notes.
The hotel provided a beamer for the laptop, and what with everyone getting me a drink, my planned 10 min speech ran to almost half an hour, as I lost my notes, had to improvise, and got a bit of audience participation going to cover up the fact that I'd forgotten the names of a few people on the photos. Everyone seemed to enjoy it, and it helped that most were almost as drunk as me. Ended the speech with a few 'from the heart' remarks about the happy couple. Most of the guests/relatives came up to me later to say how much they enjoyed it, and many loved seeing the photos especially as some were in them and had never seen them until then.
Did the Best Man bit about 15 years ago and my speech followed the Bride's father who did a full 20 minuet slide show, with photos of both the bride and Groom from birth to present day, complete with gags between slides.