I was wondering the same thing so I asked a dietitian. She said to weigh at the same time of day, preferably after you have used the bathroom, before showering, eating, drinking and before dressing. Make sure the scale is on solid ground. No carpeting, no tiling, etc. She said smooth linoleum or concrete (like the basement or garage is good).
Do not move the scale once you have located a spot because this will offset it also. Your menstrual cycle will affect it before and during also. Any medications can make a difference.
I just wanted to see for myself so this is what I did. I followed her points exactly and came up with 307.6 this morning. 2 hours later, I moved the scale, had taken a shower, drank some coffee, lazed around, I was not clothed. My weight had adjusted to 310.4. I hope this helps!
"Let no one ever come to you without leaving better and happier."
- MOTHER TERESA