1) Use of instruments with smallest precision E.g Micrometer (0.01mm) is more accurate than Vernier Caliper (0.01cm)
2) Correct eye position To eliminate parallax error
3) Reduce human error by dividing the time taken by the number of oscillations. E.g 20 oscillations timing is 18 seconds. Human reaction error is about 0.03 seconds. Dividing 18 seconds by 20 oscillations to get the time taken for 1 oscillations will reduce human reaction time by 20 times. I think this is called percentage error. Same thing goes for the measurement for the diameter of wire by winding it around a core 20 times and measure.
4) Take average measurements to increase accuracy.
