Make sure that the scales are on the same level spot. Be sure to stand in the same posture on the same part of the scales every time. If you want to monitor your weight, weigh yourself at the same time of day, when you get up or before bed work nicely. Those instructions came with my scales. They seem to work OK.

Keep this in mind from JiK.
"The real question here is how one decides whether the scale is inaccurate. A daily variation of a few pounds is fairly normal for most people, which is one reason the diet docs tell them not to weigh themselves more often than once weekly."