Get a good night's rest. Eat breakfast or lunch. This may help calm your nervous stomach and give you energy. Avoid greasy or acidic foods, and avoid overeating. Avoid caffeine pills.
Nothing reduces stress more than exercise.
An hour or two before an examination, stop studying and go workout. Swimming, jogging, cycling, aerobics. Allow yourself enough time to get to the test without hurrying. Don't swap questions at the door. Hearing anything you don't know may weaken your confidence and send you into a state of anxiety.