If you live in the US, they'll tell you you should weigh yourself "regularly" and keep a food log.
The problem is that manually keeping track of that is a pain, and so people rapidly stop doing it, even if they've got a doctor standing over them telling them to do it.
I think the solution to this is better automated tools to make this process of self-monitoring beyond trivial.
This lead me to discover the Tanita BC-500 a body fat/scale that logs its measurements to a usb stick. (Of course it's one of those only-in-japan products I keep wanting).
Not to mention people still looking at me strangely when I mutter about bar or rfid coded food packaging and storage containers, so you can have an accurate accounting of how many calories you've eaten in a day without actually having to do anything.