There's a saying that you should write about what you know, because obviously it's much easier to be more accurate, convincing and engaging when you're a slight expert on the subject. So, on the face of it, writing a best man speech for your brother might seem blissfully straightforward, but as many people out there already know: nothing could be further from the truth.
Many grooms make their brother the Best Man for the wedding - it keeps a lot of people happy and in many cases they are the ideal choice. The problem comes when you're considering actually how to attack the speech because you'll have a sudden rising panic that apart from a few incidents as toddlers and on family holidays, you really don't know the first thing about him, and how on Earth are you going to fill up a ten minute speech? I've said many times that a great best man speech is not a collection of stories and it's not an assassination, it's a warm inclusive, but very funny look at who your brother is and what he's about.
So in order to write it successfully
you need to have a complete change in your approach.
At the moment you're almost certainly looking at things along straight lines - an intro, a couple of stories and a sentimental ending. The straight line approach works only if all the guests have had frontal labotomies. You need to take the elements form those stories and match it with his character in order to make some witty observations. What does that story say about him? How does it relate to what he became? Extrapolate your thoughts and forget the detail.
Stop worrying about what you don't know about your brother and make what you don know work even better, Use the facts you have about him to generate a description which should be quite happily held together with complete fantasy. In other words it doesn't matter what you say about him as long as it's funny and has some sort of basis in fact. Dream up what you like, the guests don't really care, they just want to laugh.
A lot of best men brothers I speak to, decide to go fact finding from his friends at university, work or renting days. This can sometimes work but more often than not the information is sparse and confused, and tends to send you down the anecdote route. Nothing will sound more unconvincing than recounting a story about your brother of which you have no first hand knowledge.
The best way to attack it is to look at the following things and then weave a narrative out of them:
Remember keep it light on detail, as funny as you possibly can and within the magic 1300 word boundary, and you'll be on to a winner. And when it comes to writ in something funny you'll know when it happens - if you're unconvinced what you've written isn't that funny, then I can tell you without reading it - it's not. You can always visit the best man speech structure page for more hints and tips.