Get out of bed, hit the scale, write down the number.
This has been my routine for a few years now.
The result is a weight history going back to January, 2008 (194.6 pounds). I can tell you exactly what I weighed the mornning of the half marathon I did that Febuary (195.8 pounds) or on my birthday in 2009 (187.6).
The highest number? 197.8. April 18th, 2008... almost 40 pounds down on my true all-time high, but still a big number. The lowest? 172.6. March 2010.
One thing I have learned is not to trust a one-day number.
This last week is a great example, from Friday to today:
Now there is no way I lost 7 pounds in 6 days... not of real human fat! I biked 100k, ran 30k+6k, and watched what I ate. Optimistically, that's about 6000 calories burned - which represents about 2 pounds of fat loss.
Not 7 pounds!
So what is going on? A lot of things... that 181.6 number is artificially high because the day before was my anniversary and we ate (and drank) without restraint.
The 174.8 number is artificially low because I did my 30k run yesterday at night, then went to bed without eating all that much. I depleted my glycogen stores, which hold a lot of water weight, and then went to sleep. The result - instant pretend weight loss! I should patent this... "Lose 7 pounds in 5 days!"
The bottom line is that the number on the scale is just one piece of information to help track your health. There needs to be some context to make it meaningful, otherwise it's easy to jump to wrong conclusions.