I used to just weigh once a week, but I've started to weigh every morning - given my history of denial, I think this is a good habit for me to get into.
I still update my ticker, etc only once per week based on my "official" Sunday weigh-in, and seeing the number bounce around like crazy each day actually helps me to focus on the overall trend and be less obsessed about what it happens to say on Sunday.
This article explains the causes of daily fluctuations really well - it definitely helped me get over my frustration with daily fluctuations.
http://primusweb.com/fitnesspartner/...ight/scale.htm