make sure the bowels are emptied before going into the room. It will make a difference, as it will put you at ease.
dude, I empties the bowels 3 times before gettting into that room last year
Also, have a BIG wank before you go in. Reduces the tension.
I remember last year during the AM session, I had to pee really really bad. Took me a minute to walk to the bathroom, 3 minutes to pee, and another minute to walk back to my seat. Madness. Also, avoid too much coffee on exam day.