There is no getting away from the fact that practice makes perfect. The more familiar you become with the words, the easier it is to read vast swathes without looking at a prompt card, the pauses are all worked out in your head and you're comfortable with exactly the right pace of delivery. There's no substitute for all of this hard work and your performance will be all the richer for it...up to a point.
I lose track, and to a certain extent lose patience, with people who say they've read it out to their wives and they either didn't laugh or didn't get it. So, using this spectacularly unqualified focus group of one, the whole thing has to be rejigged. There is a huge, and I mean HUGE, difference between practicing it in front of your wife/girlfriend/whatever, on a rainy night after work and the atmosphere of the wedding cauldron. At a wedding everyone is revved up and ready to go, primed with a bit of bubbly they're hanging on to your every word and just need the slightest excuse to make them laugh. Reading it out to your partner in a much colder, more remote atmosphere is setting yourself up for a fail.
It's also saying that your humour, your cultural and social references are inadequate compared to those of your girlfriend's. Have confidence in what you first thought of as funny and go with it. There's a good rule of thumb which I always remind my clients of: if it makes you laugh the first time then that's exactly what's going to happen on the day. Over analysis of jokes, conceits and observations is completely counterproductive and all you're doing is allowing multiple sets of unqualified opinion tell you what is and isn't funny.
But it doesn't stop there because you'll make amendments and then obviously go back to the same person to see what they think at which point they'll see themselves as some sort of burgeoning critical talent and then by default have to indicate where it could be improved further. A complete waste of time.
Practicing your Best Man Speech should be a personal, solitary and rewarding experience, where on the day you alone reap the accolades. So take it from me: believe in yourself, because nobody is a better judge of what you should and can say.
