I agree with blonde--choose one time of day to weigh and always weigh then. Don't be weighing a second time, third time, etc.--you'll just mess with your head.
If you are dehydrated a bit, you could artificially weigh less. But who cares about water weight anyway? If you drink 8 oz. of water, there's half a pound right there. Add in some tea, coffee, soda, and food--well, it's easy to go up and down in the course of a day.
Just take what you weigh at the same time, under the same conditions (after you pee, wearing the same clothes or no clothes). Morning is best, I think. Then as you track your loss, you won't have to worry about whether it's a daily variation.
Remember, it's the difference we're looking at, the weight change over time, not the numbers themselves.
Jay