First ensure that you are well acquainted with what is to be expected on the piano exam. Write down the expected components of the exam.
Practice for at least an hour every day leading up to your piano exam. Structure your practice sessions by using half of the lesson to practice scales and arpeggios, and the other half to practice any repertoire that will be required.
Utilize a metronome while playing to help you with the consistency of playing the technical exercises. Work on the exercises required for the exam slowly and increase the speed as you gain technical skill.
Memorize the material that you will have to play for your exam. Then you will not have to reference the music while practicing. Record yourself playing to gain a more objective idea of how you will perform during your test.
Ask a trusted friend or tutor to help you and give you a mock version of your piano exam and grade you. Relax before your exam and run through the material once before having to perform the material.