SparkPeople, where I keep my online food log, had a great article called "Lose Twice the Weight by Tracking Your Food". This study found that people in a weight loss program who logged their food lost twice as much as those who didn't.
I am a firm believer in food logging. And having some accountability with a friend or trainer checking the logs makes it that much better. I'll admit it took me some time to really integrate it into my life and to pay attention to serving sizes. Now, I will sometimes even log my dinners before dishing mine out so I know what an appropriate amount is, and I use a postal scale to measure it out. Not obsessively so...!