The Health and Fitness Blog (dead link) links to an article that discusses daily weighing of oneself and weight loss. They list these weigh in tips:

• To minimize variation, always weigh at the same time of day, such as in the morning just after having used the bathroom.
• If you are weighing yourself daily, understand that your weight will vary day to day. It’s the trend that’s important. You may find it useful to make a graph of your weight.
• Put the scale on a flat, uncarpeted surface so the readings don’t wobble.
• Use a scale that is consistent, giving the same weight when you step on, then off, then on again. That’s more important than the type of scale you buy. Use the same scale each time.
• Don’t get fixated on the scale. Monitor your body change in other ways, such as the fit of your clothes, a tape measure or how you physically feel.
• To the best of your ability, try to gauge how the weighing makes you feel, and if it is reinforcing – or undermining – your efforts.

The only thing I would worry about when weighing yourself daily and trying to lose weight is to become obsessed with it and to forget the healthy ways of losing weight (such as healthy food selection, portion control and exercise) and to concentrate on the unhealthy ways (such as not eating enough to balance your diet, anorexia, bulimia) to make the numbers go down.

However, I think that healthy people, whether they are trying to lose weight or not, should weigh themselves every morning. Even though I never want to lose weight and am often actively trying to gain weight, I weigh myself almost every day that I am not travelling. If I am getting sick, then my mean weight will usually drop rapidly over a couple days before I actually begin to feel sick, which gives me a heads up that I should back off on my training and make sure that I am getting enough vitamins and minerals and water to fight off whatever bug is trying to take up residence in my body. It also lets me know how my training is going; any sort of rapid change in weight or heart rate can be a good indicator that you are training too hard and need a rest day. If you log what you are doing, then you will quickly become used to what your body does and can tell when something out of the ordinary may need attention. Not that something out of the ordinary will always need attention, but forewarned is forearmed.

Some tips that I would add to (or expand upon) the ones they listed above are:

• Always weigh yourself first thing in the morning after your bowel movement (if you regularly have one first thing in the morning) and before you have done any sort of physical activity or eaten anything. This gives you a control and allows you to compare your weight from one day to the next. Your weight can fluctuate five or more pounds throughout the day.
• Always weigh yourself naked or at least wearing the same clothing for much the same reasons as above. If you wear the same bathrobe or pajamas every morning, then it won’t matter, but if you wear a 4 pound bathrobe one day and a 4 ounce pair of skivvies the next, you can’t really compare the numbers.
• Do not zero the scale. Let the numbers be “wrong”. The idea is not so much to achieve some arbitrary number on the scale, it is to track the trends from day to day and week to week. If you know that the scale is going to give you an innacurate weight, you won’t care as much, even if you know that it is 5 pounds heavy or 2 pounds lighter than it should be. Just make sure that you your guests or family don’t accidently zero it for you.
• Write your weight down every day that you weigh yourself. If you put it into a notebook, you can flip back and forth. If you put it in a spreadsheet, you can make graphs that can show you trends. Either way, you can look back and actually glean some useful data from it to help determine if a swing is normal or not.
• Don’t worry if you weigh more or less than yesterday. From one day to the next, given you follow the above and you are healthy, you should be within a pound or two of the day before’s measurement. If you are more than 3 pounds lighter, then you may be getting sick or may need some extra rest. If you are more than 3 pounds heavier, then perhaps you forgot to empty your bladder or your bowels first.