The PreventScripts guide to scale accuracy

Place your scale on a hard and flat surface. Your scale has sensors in the corners. Placing your scale on carpet or other uneven surface can uneven pressure on the sensors and cause a faulty reading. 

Weigh yourself at the same time each day. After your first morning trip to the bathroom, weigh yourself with an empty bladder.  This will minimize the normal shifts in your weight that come daily with drinking and eating. Compare apples to apples for your most consistent result.  

Tap the scale with your foot to re-zero before weighing in.  Your scale will naturally calibrate itself and the display will automatically show 0.0kg/0.0lb when the calibration is finished.  Then step on the scale for weighing.