So many best men that I write for all say the same thing: 'I haven't got anything to say!', and this can be for a whole variety of reason. Sometimes you really don't know the groom that well, you maybe haven't hung out with him for years, maybe he's really boring, or maybe he's so naughty there's nothing you can say.
At this point I always advocate a little bit of fantasy to glue the whole thing together, I'm not saying invent stories to take the groom apart, but merely sprinkle some fantasy dust on it to make it funnier. However, it should be all rooted in truth.
If he wanted to be a spaceman when he was younger, then I would use that to create some sort of fictitious application to NASA, or if he keeps crashing cars, then link that back to the fact he's been born with a big head and this impairs his ability to move and respond in a usual fashion. You can then stretch out both these types of scenarios to write the remainder of the speech - what life was like for a failed spaceman from Didcot. As long as it's funny and related back to his character, then all is good.
However, whatever you do, don't be tempted by the fake telegram business, which at best is nonsense and at worst extremely unfunny. There are Chelsea Pensioners that are predated by some of these gags, and as most best men live in fear of delivering a gag and it not working, then this is shark infested waters. Everyone in the room will have heard these before, and they've all had to go through the motions of laughing gently whilst sighing and desperately thinking of how to get more wine to the table.
These telegrammes normally come in the form of 'the girls from the Ping Pong Strip Club in Bangkok say hi'. Whichever way you look at it, it's seriously unfunny. Maybe when the first guy said it in 1908, it had the necessary comedy clout, but the crucial thing with humour is exposure. The more you're repeatedly exposed to the same gag, the less funny it becomes, and the bottom line with a best man speech is...it needs to be funny.