I have been to many weddings, and I can say with the utmost confidence that the part of the night I look forward to most is the best man's speech. The best man, be he friend, brother, cousin or otherwise, knows the groom like no one else present. He can tell you a distant chapter of the love story you didn't know, but are dying to hear. Because nothing warms the hearts of a room full of people drunk on love, like a bit of “bromance”.
If, like many, you're struggling with the pressure to come up with the words for your own speech, read on good sir! Alternatively, if you're planning on winging it, I'd suggest don't, unless you're widely renowned for thinking on your feet. It's an honour to be selected for best man and you should thank the lovely couple with words that show thought and effort.
It may sound pedantic, but the best speeches have structure. Just like your essays in school, a good speech should have a beginning, a middle and a conclusion. Choose one strong idea supported by several points. It's better to represent one story well, rather than trying to cram every childhood adventure into five minutes.
Avoid “in” jokes. You're making a speech to an entire room of guests, so your job is to include them in the funny bits and share with them the stories, so that they can relate and react with the couple.
Remember your manners! We all love embarrassing our mates, but choose your moment and your audience carefully. Nana doesn't need to know about her grandson's wild antics in Mexico, or his little black book...or several other things for that matter...
Keep it short and punchy! Generally people don't remember what is said in a speech, only how it made them feel (this may or may not be a result of free booze on tables). Go for emotion over detail and keep it simple.
Be genuine and natural. Your speech doesn't have to be revolutionary or hilarious. It just needs to come from the heart. Take at least a few days to write it. You'll find you make a lot of changes and delete unnecessary blather if you sleep in between edits.
Other than that, be sure to thank the guests for coming, point out how enchanting the bride is, and you've got a winning speech right there. Deep breaths, talk slowly, you'll be right mate...Cheers.