Oddly enough, when I got the foodblogging bug, I actually lost weight and kept it off! In hindsight, it shouldn't be a huge shocker, since I've been doing a better job taking notes and keeping track of what I eat. And of course, even less of a surprise that most sensible diet plans do strongly encourage (if not outright mandate) participants to keep a food diary of sorts.
Of course, there are those who insist they keep a good tally of what they ate "in their heads." But, time and again, studies show the human memory to be fickle, unreliable and malleable -- and these are studies on life-and-death situations of violent crimes and murder trials, let alone the everyday mundane going-ons of "what did you have for lunch two days ago?" (Come on, can you really tell me off the top of your head?)
I'll even take it one step further and say that, in terms of food, people tend to be selectively forgetful -- subconsciously burying recent splurgy meals, indulgent snacks and satisfying a craving while "remembering" how good they've been this past week.
I'm not saying you need to scrutinize and catalog every single calorie that passes through your lips (I sure don't,) but at least keep a fairly accurate record of what you are eating -- the more info you can include (portion sizes, calorie counts, other relevant nutrition data) the better.
And with so many free services out there, it's never been easier to spend a minute to jot down what you ate. You can set up a twitter/tumblr/blog account to text into (even make it private if you don't want it to be publicly known), do it the old-fashioned way like I do with a pen and notebook, or even just save your receipts when you're eating out, which is when one's most likely to splurge anyways (it's great way to keep your budget in check too.)
Finally, do a review every now and then and see if you've made progress or if there's room for improvement in your dietary habits. Then make those gradual changes and you'll feel and look better before you know it!