The Best Man should take care of any problems on the big day, the Groom will be nervous and hence his logic and reasoning may suffer. Make sure you have the numbers in your phone; chauffers, hair dressers, the wedding party, minister/priest/scientologist, cake maker, florist. Back-up plans are a good idea. If you're not sober enought to drive, keep in touch with someone who is, last minute errands are a certainty. People get drunk at weddings, you should be on hand to break up the ensuing fights and arguements. As well as your speech, you'll need to read out the cards, so remember to get them from the groom.