Neck: To stretch your neck, flex your head forward/backward, side to side and look right and left.
Shoulders: Roll your shoulders forward for around 10 times, then backward.
Arms and shoulders: A good stretch for your arms and shoulders is to brace your hands on the edge your desk, each about a shoulder width away from your body. Twist your hands in so they point towards your body and lean forwards, hunching your shoulders.
Wrists: Roll your wrists regularly, around every hour or so. Roll the wrists 10 times clockwise, then 10 times counterclockwise.