*Plan it! Don't make it up on the fly. Even if you're the most quick-witted and glib person around (and maybe you are!), you can't rely on chance; anyone's mind could go completely blank when it's time to make a speech.
Even if you don't feel like writing the whole thing down, at least take the time to jot down a few key points and phrases.
*It's okay to be funny, but be serious. Sure it sounds like an oxymoron, but it's still important. If you're not a little bit funny, you could wind up boring the wedding guests.
However, it's imperative to remember that your best man speech is a tribute to a couple's new life together. You can be sure that the two of them take their marriage very seriously, and they want to know that you honor that. Sarcasm might be funny and enjoyable in the right circumstances, but please remember that it is NEVER appropriate in a wedding speech.
*Keep it brief. Please don't go on and on. Before the big day, practice giving your speech in front of another person. Watch them. If their eyes glaze over, you need to trim down your speech.
*Go easy on the anecdotes. Steer clear of stories about your friend's old girlfriends, and don't go telling the guests about embarrassing moments that could make your friend's new wife want to clobber you. If your buddy's wife doesn't like you, don't expect to see much of your friend after the wedding is over.
*Share something you like - something appropriate! - about the bride. Don't forget, you want to be endearing yourself to her.
*Above all, be sincere. You were chosen as best man for a reason. Keep that in mind and share in the joy of your friend's happiness.