Despite well catalogued best man speech disasters on the internet, there are still a legion of men around the world convinced that they can stand up, and knock out a great speech from the depths of their mind. This is only ever Route 1 to disaster. If you've ever witnessed a really effortless, very naturally delivered speech, it's because it will have been weeks in the making. Most people aren't required to be seasoned raconteurs for their day job, so the idea you can transform yourself from IT support professional to a public speaking superhero in a matter a of minutes, just doesn't stack up.
Any great speech needs a structure, well planned jokes, an engaging beginning and a meaningful end. The attempts that I've seen when best men get up and wing it on the day, are only ever a meandering, jumbled mess, and test the patience of the guests beyond reasonable limits. If you go down this route you're also going to have to employ the 'stories' method of telling you best man speech, and you simply won't have time whilst delivering it to construct any clever jokes or observations about the groom. And it also means timings are an issue. Best men who decided to wing it on the day either crash and burn in a matter of seconds when they realise just how far out of their depth they are, or they're so busy telling lengthy stories that they end up with a 30 minute epic.
The groom has asked you to do a really important job on the day, and you owe it to him to give the best shot. This isn't a plug for my services, but unless you're some kind of freak of nature, you're never going to be able to stand up and celebrate him in a really entertaining, funny, meaningful and memorable way. You might think it will look cool to do it off the cuff, but my advice is to start early, plan it and practice as much as you can, and smash it on the day. There's nothing cooler than that.