A trainer friend of mine did a study (totally unscientific, just for her own personal info) about 10 years ago when those scales first started becoming more mainstream. She compared the scales, the hand held machine (Omron, I think?) (similar to the method a scale uses) and calipers. All in all, the best scale performance came when she did the following:

1) get up 3 hours before your normal rise time (set an alarm) drink a 12-16 oz glass of water and go back to sleep.
2) get up at normal time, pee, and weigh.
3) make sure this is not the week of your period, that you didn't eat a huge salty or fatty meal the night before, and that you aren't super sore from a workout.

I've also heard that plane flights within 24 hours can also screw with the numbers. Basically, your body water content is what most drastically affects the numbers. This is why they advise against an early morning weigh-in, you are dehydrated after a night of sleep.

All of the above aside, if you get up at the same time and do the same things every single day...then weigh then and make relative comparisons. By doing all the above to get as accurate a reading as you can just makes it harder to get repeat measurements keeping all other variables the same.

Oh, and I've always found that the scales predict way high for me. I carry ALL my weight in my lower body. The hand-held and the calipers gave me basically the same number but the scale was 3 - 4% high.