Relax - Remember that everyone who's listening to your Best Man speech are out for a great day, and they want everything to work for you. They don't care if you're nervous, or if you fluff your lines - these things happen so just laugh it off and move on.
Learn your speech - by reading it over and over again. After you've read it 7-10 times, try and read it without looking directly at the page. If you forget something, check what it is and then start again. Keep doing this and soon start to remember it without reading off the page. This method takes several hours, but works.
Take your time - If you sound like you're speaking too slow... that's just the right pace. The slower and clearer you deliver the jokes, the funnier they will be. Listen to your favourite comedian - they shout out the punch lines of jokes slowly to make sure you understand them!
Don't be afraid to pause - When people laugh at something you say (and they will), pause and let them enjoy the joke before moving on. Watch the best comics in action. After a joke or routine, they'll pull a face and wait for the audience to stop laughing before moving on.
Watch your drink - Have one or two drinks to calm the nerves before the Best Man speech, but no more. Any more drinks, and you'll start to forget what's in the speech and how to deliver it. Remember this point!
Talk clearly - make sure that you're loud enough for everyone to hear. You don't need to shout, but talk just above the level you would normally. If you're offered a microphone - make sure you take it. Your lines will sound best delivered loud and clear.
Smile, be confident - use your hands to explain things as you talk. Imagine that you're explaining something to a bunch of your friends in the pub. Because on the day, that's just what your audience will be.
Feel free to change it - when you perform a Best Man speech, don't worry about following the script word for word. Changing the odd line to make it sound more natural is fine - as long as the main idea (and the punchlines) are the same.
Have fun - Remember, this is your moment to shine. With a great Best Man speech and a relaxed clear delivery, you really will have a great time!