What Mal says is true - you need to stick to the same part of the day, that way you are checking how much lost rather than a specific weight.
I recently got myself new scales because I thought my old ones were playing up, and I don't like them because they make me 4lbs heavier, so I use them as a way of callibrating my old scales. What I'm actually checking is how many lbs lost in total, so I figure that my new scales would have started me at 4lbs heavier and my total weight lost is still the same.
As for me, I weigh myself naked, first thing in the morning, after I've had a bowel movement. Sometimes it can take a glass of water to get that movement, but it can make a 1/2lb difference
