I've just completed Best Man's duties, which of course involved the customary speech, and I am able to offer the following advice.
Know your audience - no crass jokes that may offend.
Write your speech down - as stated earlier rehearse it and if unsure on the day read it verbatim. You'll find after a couple of minutes it will flow without too much looking down at the page. (One other tip is too mark one's place with your thumb whilst your gaze is away from the page to assist you to quickly find your place.
Focus on one person - pick one person out from the crowd and pretend to address them personally, occasionally glancing across the room for effect. This will regulate your speech to the same tempo as a standard conversation.
Pause regularly - helps to control your breathing and emotions and judge the audience's reaction.
Remember the maxims - beginning, middle and end!
Keep back a banker - save your best one-liner (proven) as a reserve in case of emergencies or when the audience wanes, and try to involve someone in the audience in your speech, so that some of the focus is distracted from you.
And finally CHEAT! - I managed to find some good examples of speechs on t'internet, and was able to bastardise them to suit.
Good luck on the day, but remember to enjoy it. Once you get into the swing of it you'll be OK. If you look and sound nervous the audience will cringe. Prior planning etc.... always prevails!
