After thinking about why I always ate a bit too much, I finally realized it was a problem with my physiological empty/full gauge. If I eat until I feel "full" I've probably eaten too much. And, worse yet, I end up feeling sluggish for an hour or so after eating. You know, the "food coma."
Habit #3 is about resetting your notion of when to eat (or stop eating). The easiest way to say it is "eat when you're hungry, stop when you're not." Notice that this says nothing about feeling full.
This is the single most difficult thing to do. If you're like me, it means breaking 30 years worth of training your body. But after the first few weeks, you'll start to find that the "not hungry and not full" feeling starts to seem normal. If you keep a running tally of your food intake during the day (habit #1), that'll make it a lot easier to know when you should stop.