Track your food. Write it down. Count calories or macros or nutrition or portions or whatever system works for you.
I have used Weight Watchers Online with great results. With WW you don't count calories (well you really do, but it's all under the hood and they do that part for you to keep you at a healthy goal). With WW they tell you instead how many POINTS to eat. (You must have so many fruit and veggie points, so many meat points, so many carb points, fat points, etc.)
I found that incredibly helpful when I started this journey and in fact used it for the bulk of my weight loss. It guided you through what is a healthy portion size and really made you think about how much of each type of food you were consuming. I found I was eating a LOT of processed carb crap and very little veggies.
The bottom line is there was an online tracker, and an Android app (iPhone too) so that wherever I went I could drop in that meal and it would keep a running total. In that way I knew when I had to stop for the day. WW was also great because it is designed for people who make mistakes and missteps. They built in daily and weekly overages, knowing full well that at some point you'd blow it. But it was okay because they accounted for it with 35 "anytime" points to cover you. Brilliant!
Now I use My Fitness Pal: http://www.myfitnesspal.com/ (You can add me as a friend if you want...my username is Carlyweb and I keep my entire food diary public. How accountable is THAT?)
Every bite of food I eat gets recorded (most days) and it keeps me honest with myself.
It is SO easy to lose sight of what a portion size should be. It is so easy to forget about that granola bar you ate this morning or all that half and half you put in your coffee this afternoon and before you know it, you are hundreds of calories over. It only takes 3500 unaccounted for calories to gain a pound and frankly, I can do that pretty freaking quickly!
So today's tip is TRACK YOUR FOOD. You don't need a fancy system (my fitness pal is free though and so easy!) Even if you just keep a journal, diary, notebook...something.
And write down every nibble. ONE bite of fudge is STILL calories. Just because you don't eat a whole something-or-another doesn't mean it's calorie free.
Be honest with yourself, because even if your mind deceives you....the scale will not.