I've decided that the most reasonable way to track progress against one's weight during a weight loss program like this is to focus on the low numbers, and to measure yourself frequently. I try to weigh myself at least four or five times a week, in the morning right after I wake up, before I've had anything to eat or much to drink or get dressed, and then I only really pay much attention to the number if it's equal to or less than the previous number. As soon as I start my morning workout, the numbers become meaningless. I'm wearing clothes of an unknown weight soaked with an unknown amount of sweat, and I've probably consumed a ton of water at that point (or maybe not, depending on how thirsty I was). No scientist is going to put much stock in those numbers, at least compared to the somewhat more controlled numbers that you can get by weighing yourself each morning. Those numbers still vary quite a bit, but they certainly seem more meaningful.
The reason I only focus on the low numbers is that I've come to admit that the high numbers aren't because I'm magically beginning to gain weight following this low-calorie, low-fat, low-carb, low-sugar diet and exercising like a crazy person—they're because I'm a complicated machine and digest and, um, excrete food at rates that are somewhat random. Now, once I start to return to a more normal lifestyle, or even around the time that I'm in Nebraska for Christmas and I'll probably regain a few pounds, I'll have to start paying attention to the numbers when they go up, but I still think that the most consistent and useful weights to pay attention to are the low ones, because those are the ones when you aren't full of water and... other stuff.
A couple weeks in, when I started to see the numbers occasionally go up instead of the free-fall I was in when I started, I had to argue with myself a bit to come to the conclusion that only reporting the lows on my "bathroom scale" on this blog was reasonable, and not just fudging the numbers to make myself look better.
This advice that I've settled upon for myself is actually in direct contrast to the advice I've gotten from multiple alumni of the same weight loss program, oddly enough. Universally, I was told not to weigh myself at home, and to only weigh myself once a week, with my dietitian. The reasoning for the conclusion was the same as the reasoning for mine—because weights vary wildly from day to day. I can see the wisdom in that—you don't focus on the small day-to-day variations because there are none, and you're likely to be at a similar body composition every Monday at 11:30 am—but I like my way a lot better.
I've noticed patterns, too: my weight is highest in the middle of the work week, and the lowest on Saturday and Sunday. I'm not sure that I really have an explanation for that yet.