Try reciting all the subway stops on your train, or all the exits on the highway ride home or mile markers in the race. After your trip through the toilet line, have faith that once the race starts, you'll forget about that bladder. This gives the water time to work through your system. Have another cup or so just before you go to line up. It'll take about 15-20 minutes for your body to absorb that last drink.
Sports drinks are also okay.
Avoid fruit juices that can cause stomach cramps. Once you are a little sweaty stretch out lightly. Some years the Mini features a guided group stretch. It's a nice touch and gets you in the group motion mode. In case you weren't completely prepared, here's a heads up - there are a few thousand women at this event.