I have never been personally selected for random drug testing at competitions (knock on wood), but through one of my old synchronized teams have had the, erm, "pleasure" of undergoing similar drug tests.

Once you are notified by an official "escort" that you have been chosen for a drug test, you must sign a Doping Control Notification Form. From that time you have one hour to report to the testing station, during which time you may not leave the sight of the escort. However, you should report to the station as soon as possible after being notfied.

Upon your arrival, you are encouraged to drink fluids, preferably from sealed containers which you open yourself. Since caffeine is restricted, water or juice is probably a safer bet than, say, chocolate milk, cola, or coffee. Even though you need a rather large concentration in order to test "positive," it is probably still better to stay safe in that respect.