You'll be expected to spend at least part of your time on the mic poking fun at the groom. That said, wading in early with ribbing and put-downs is a mistake. Before you get your claws out, you'll need to have the audience on side and rooting for you. Then, when you do unleash your cunningly crafted character assassination they'll be right behind you and ready for anything.
The best way to get a wedding reception crowd on side is to say something nice about the bride. Everyone will agree that she's beautiful. They're all but legally required to do so. You could also add that it's been a wonderful wedding so far.
Only the most curmudgeonly of guests would dare to disagree:
"Paul, you have really scored in marrying Linda. You've found someone who is attractive, funny, smart, loving and caring. And Linda, you have found … well, you have found Paul. But seriously, I think you've made a wonderful choice for your first husband."
It's not a requirement or a duty by any means, but if you wish to continue with the kind sentiments you can choose to refer to the bridesmaids, or to the broader spread of attendees with a more general thanks for coming line.
"A big thanks to the bridesmaids. Now I know what you're thinking, but you'd be wrong. It is apparently quite difficult to cry, look pretty, and hold flowers all at the same time, especially for an hour. So I'm sure you'll all agree they've performed such a role splendidly. And such a coveted position no doubt comes with some inherent rivalry, mostly about who will get the first dance with the best man."
As we've said in Create a Compelling Opening Line, making yourself the butt of the joke can be a great way to get the audience to warm to you:
If you were uneasy about the responsibility …
"Now I know I wasn't Paul's first choice as best man, or second or third, but the only suit left at the hire shop was extra large. I proudly stand before you."
On the subject of your duties …
"When Paul asked me to be his best man, I discovered that I had three responsibilities. First, I had to make certain that the groom was properly dressed for the ceremony, and to that I say, Paul looks fantastic. The second responsibility was to get the groom to the ceremony on time, and to that I say, Paul was at the altar exactly when he was needed. And third, it was my job to guarantee that the groom arrives at the ceremony totally sober, and to that I say… two out of three ain't bad."
On the difficulties of preparing your speech …
"Never in a million years did I think that Linda would actually go through with this and marry Paul, and I have to admit I'm utterly speechless. Seriously, I didn't write one."
Another way to get the audience on your side is to draw on common ground, and the one sure fire thing you all have in common is that you're at a wedding. Provide them with an amusing insight into any number of the day's anxious moments, comment on the service, make an observation about the ushers, or say something amusing about the wedding gifts. Everybody present will be able to relate to the joke.