He's your best mate, you've known him forever and he's a laugh a minute. That is what the groom sees when he looks at the best man. What the bride sees might be something entirely different.
Theorectically, the best man is to the groom what the chief bridesmaid is to the bride. He helps him get ready, carries back up supplies for emergencies and is generally the first rock of support on the big day. In reality, we all know that best men are liable to play pranks and make embarrassment for you and other people.
How can you mitigate as many problems as possible? This starts with the choice of best man. Pick someone who knows both of you really well. This way, if he wants to play jokes, he'll know how far he can push things without getting anyone upset. After all, sometimes the jokes can add the high spirits of the day.
You also want someone who can act and look the part. There's no point in picking someone who is uncomfortable in a suit or nervous looking after crowds. A best man needs to be a people person; someone who can recall information on demand and be a help to other guests. Whether this be giving directions to from the church to the reception or knowing where the toilets are.
In truth, I cannot recall the last wedding I attended where there was just one best man. Often the groom has more than one brother, or a friend from years ago and one from recent times, and he cannot choose between the two. This was typified in the Friends episode where Ross can't choose between Chandler and Joey for his wedding to Emily.
The other important job a best man often has to carry out is the best man's speech. Whilst the main point of the speech is to poke playful fun at the groom, it's good if he (the best man) knows where to draw the line. Experience in public speaking never hurts either, although it isn't essential. If talking in front of an audience of dozens is not something the best man has ever experienced before, it is advisable to have at least one practice run. This is true for all those planning to give a speech on the day.
Worst comes to the worst, your best man would have a long way to go before he creates the same level of chaos as at this one. Good luck!