Plan. I menu plan for the entire week in advance. I pack lunches/snacks in advance. If I'm sometimes too tired to cook, I have healthy quick meal staples on hand. I keep dried fruit in my car, I keep soup at work. I plan on taking snacks to the movies. I am rarely without healthy food choices at my fingertips.
Forgive myself for going off plan temporarily. The world is messy and complicated and sometimes wine and cheese and crackers will happen at a work event. I used to beat myself up, I used to call the whole day wasted and eat a cheesecake. Now, I forgive myself and immediately move on.
I always stay on plan when I start the day with exercise. I don't know if it's because I feel so good or whether I don't want to waste all that effort and time I spent in the gym by eating what I shouldn't, but it works.
I always stay on plan when my plan is well thought-out. If I've gone to the store for all of my groceries, made my meal plans, and laid it all out in fitday, my day usually goes without a hitch. If I'm going on the fly, though, it rarely does...especially if I go to the store to buy stuff for dinner that night between 2pm and 6pm and get tempted by one of the many sweet temptations at my local grocery (see "I NEVER stay on program when..." response).
Plan, plan and plan some more. Which means setting up menus for the week and doing the necessary grocery shopping. Taking the proper things out of the freezer for defrosting (such as chicken cutlets). Making yummy food is also a key to staying on track for me, which is indeed part of the planning process.
However, my biggest thing is writing in my journal. I write every single thing I put in my mouth in my journal. It keeps me accountable and it helps me when I do have the munchies because I can look and say I have eaten plenty and don't need more.
Like Meg, I always stay on plan when I get in my exercise. Somehow, it really focuses me on what I'm trying to accomplish, and all the eating issues just fall in line.
Ok, I know everyone has said this before, but it is SO true. If I don't plan ahead (and have an emergency meal set aside just in case the unexpected happens), then I won't stick to my diet. I plan my menus in advance and go to the store on Sundays to get all the ingredients I need. I make my lunches the night before. Because I work full time and go to school half time, there are two days a week that I don't get home until 10:00 p.m. On those days, I have to make sure I have two meals and three snacks packed or I get hungry and head straight for the chocolate.
When I have choices! Sometimes I'll plan a meal, and then just don't WANT it anymore.. so I always make sure I have at least 3-4 choices of what I can eat. (also when the dishes are done.. so much easier for cooking!)