A good best man speech, should be about the couple from a third party perspective. I'm assuming you're the groom's friend, so give a introduction of yourself (including how long you've known him)...and talk about lame/lonely/losery he was before he met her. Toss a few harmless jokes at the groom. Then talk about the ways he's changed for the better since he met her...and how you've watched them grow together, and want to wish them the best of luck as they begin a new life together. The best advice is keep it short, keep it clean. I was once at a wedding where the best man told a pair of anecdotes about the groom's masturbatory habits, and the bride's promescuity. Not a good idea.