When measuring, make sure that the sensor is securely attached to the finger, and that there is no excessive pressure.
Wash your hands. Women should clean their nails from varnish. Warm your hands if possible, as cold fingers can cause inaccurate readings.
Try not to move during the measurement, because pulse oximeters are sensitive to bright external light and vibration.
Keep in mind that during and immediately after physical exertion, a slight deviation in the level of oxygen in the blood from normal levels to a smaller side is acceptable.