I agree with Suzanne. Whether you are calorie counting, doing Slim-Fast, WW, or whatever you are doing-I believe that it is of the utmost importance to keep a journal of what you eat and drink every day.
It does a few things-it helps you realize exactly what you are eating each day. Even if I am over my calories for the day-I still write down whatever it was that put me over. It helps me realize later in the day-"you know I haven't had that much water/enough fruit and vegetables" or whatever during the day...it keeps me on track.
After doing it for a few years-it is second nature to me-just like writing a check or sweeping the floor. I don't think about it-I just do it.
Most successful losers and maintainers (of weight loss) keep track of their diets and exercise in some sort of journal.
I use a simple tiny sized notebook-and use a page for each day.
I have also found that staying on plan can be as difficult of as easy as you make it. It is all in the attitude. If you tell yourself that it is too difficult to stay on a plan, or too difficult to keep a journal-then it will be. A positive attitude helps so much.
My husband is the bacon/soda loving KING. His family has no fresh fruit in house EVER and everything is deep fried or loaded with sugar. It's like a fried chicken and cherry pie bomb went off in their house.

We have to go over there for family functions-and even spend the night during the holiday seasons-and I have had to work my way around it. I bring a veggie or fruit tray to a family dinner...they have a separate fridge in their garage for the 80 cases of Pepsi and Mountain Dew they have-and I nagged them to get some bottled H2O and Diet Coke in there for me.
If we spend the night-I take a little soft sided cooler with a couple pieces of fruit, a meal replacement bar (not Slim Fast-but higher quality bars like Clif, Myoplex, etc.) and a few other small portable food items with me. If they make something for dinner that is too unhealthy for me even in small portions-I have my own food to fall back on.
Planning and journaling-in my personal experience-is a huge part of succeeding.