Okay, folks, gather round and da perfesser will share with you the secrets of weighing yourself scientifically.
First, if you are obsessing with the scales, you may want to put a planter or something on them so you don't notice them sitting there pleading for a tender footstep. Or you could do as Kath suggests, and see how far they can fly under their own power ...
OK, we're past that little problem. Next thing to understand is that there is only one time during the day that will give you an accurate weight that you can compare from day to day. That time is after you get up from your good night's sleep, whether that's in the morning or mid-afternoon for graveyard shift workers. Your body has processed everything you've thrown at or into it. There's nothing left in your pouch. Empty the trash that it's set aside for you as your first item of business.
This puts you at your minimum weight for the day. You will never weigh less than you do just after you get up and come out of the bathroom. So why weigh when you're guaranteed to weigh more?
So get on the scale and let it stabilize. Wear the same things (or lack thereof) when you weigh so you don't introduce another daily variable here. If you have one of the old spring scales whose main virtue is it didn't cost much, you get what you pay for. Springs aren't really all that accurate, but you can still get consistency from reading to reading if you're doing it at the right time each day and are careful to get your feet in exactly the same position every time (it makes a difference on our old Health-O-Meter spring scale!) I now have a set of balance beam doctor's scales that let me slide the weights to get a stable reading. You can't get any more accurate than that! The new electronic scales give pretty good results, too, as long as the battery is fresh.
Record your weight on a chart so you can see the trend of the data. The direction you're heading is far more important than the actual reading. If you're stalled, it's easy to see that you're not gaining, which is good. If you have a one-time bump up of a pound or two, you will quickly see that you got over it. If your weight starts trending up, why? Might a food journal help?
Look for where the curve is heading more than where it is. With body weight, it takes several days to see a significant difference, so don't take each day too seriously. As long as you are sticking to the rules, you'll move in the right direction.
Once you weigh, that's it for the day! From that point, you will be taking weight in, holding it, and letting weight go in a continual process. Water weighs 1.1 pound per half-liter. If you drink a bottle of water, your weight will go up. (Your goal is to drink at least 4 half-liters per day - figure it out!) You lose this weight through sweat, breathing (water vapor) and excretory processes. Trying to weigh at any time during the day is futile unless you are totally sure that you have gotten rid of exactly the amount you have taken in. Then there's food, which weighs something, too. Weighing when you get home from work gives you a number that includes your accurate weight - PLUS everything you've eaten or drunk that day - PLUS whatever you are wearing that you weren't when you weighed that morning - MINUS the amount you have lost through your pores, breathing, or in the bathroom! You would have to have lost a lot of real weight during the day to even weigh the same as you started!!
So there you have it. Weigh early, weigh once, plot it, forget it. If you are going to obsess over something, that's your personal choice of entertainment, but if you obsess over multiple weighings, that's all it is. There is only one standard time to weigh, and there can be only one valid weight taken. Look for the trend, not the daily differences.
Oh, yeah, make sure the scale is on a hard surface when you weigh. If you put it on a carpet, you introduce a variable, as the carpet will absorb some of the weight as a result of bending the fibers and you will get neither consistent nor accurate results.
Here's to happy losing!!
CT
The science of weighing ... 
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