I guess I'm going to be a dissenter on this one. I definitely weigh myself everyday and enter my weight into DietPower daily. Luckily, I have a Tanita body fat scale, and based on the body fat reading, I can usually tell if my weight is up because of water retention or some other fluke. I tried weighing once a week, but I found that weighing every day keeps me honest -- it was too easy to fudge a little thinking I'd "make it up" later in the week.
The key thing is that you have to find out what works for you. If you're getting too discouraged, definitely weigh once a week or month, but if you're like me, you'll never be able to keep yourself off that demonic scale.