Oh man, I'm glad I'm not the only one who has problems with the scales.
I would love to get to the stage where I could just have my weekly weigh ins at weight watchers and be done with it but I do it at least once a day. It's so depressing when I see a gain, like this morning I am 800g heavier than yesterday, for no rhyme or reason.
It does tend to set the mood for the day and I have to try and not let it get me down. At least by this stage I know that it does fluctuate and it will probably be gone again tomorrow, but the best would be to just get to a stage where I could weigh a lot less frequently. I would say that once or twice a week would probably be ideal.