I'm overjoyed to learn that Prince William has asked Prince Harry to be best man at his wedding. I was the best man at my brother's bash, and one day I hope he'll do me the honour of returning the favour.
It's an onerous task, no doubt about it. I strongly advise Harry to veer towards the sentimental - no gags about that time that Kate had to be taken home in a helicopter because she'd clobbered a servant with a Bolly bottle and puked vintage champagne out of her adorably common little nose, for instance.
Chuck in the odd cheeky anecdote, emphasising the loveable-oafishness of the groom. Frankly, Wills looks like the kind of geezer who spent a considerable portion of his youth slamming his naughty bits in a door, so that might be a fruitful line of inquiry, but there's no need to be vulgar. Only use photographic evidence if it's tasteful.
Thank the in-laws, especially if they're paying for the gig, and try not to be too offended if the bride's father uses the wrong spoon on his swan soup or something equally ghastly. Also, be sure to toast your own Dad. I doubt Charles will have invited him after that whole "secretly banging Diana" malarkey, but it's the thought that counts.
Don't forget to chuck in a few near-the-knuckle chucklers for Prince Phillip, but nothing too racy*. Anything that starts "A Pikey, a Pole and a Chinaman walk into a bar" will probably hit the funnybone, but definitely, definitely no comedy armbands or salutes this time. Keep that to the stag do, when the press aren't watching.
If there's one thing I regret about my effort, it's that I never got the opportunity to slip my elephant impersonation into the speech, because kilts don't have pockets. It'll be tops 'n' tails for the royal bash though, so carpe diem, your Royalness.
And finally, remember the golden rule - everyone will be pissed up, rowdy and jammed with goodwill, so enjoy yourself. This is the only chance you'll get to have a bit of fun at everyone else's expense, so be sure to make the most of it.
Best of British to ya, your Worship.
* of course, "-ist" is perfectly fine.