Commit to making good choices and honor that commitment, every minute, no matter what's happening tonight or what you did the day before or that morning.
cut out the foods you tend to eat too much of. for many, its soda, or cookies, or fast food. for me its pasta, which isnt a bad thing, except i always eat 1/2 a pound of it at a sitting,and i would eat it several times a week- with heathy, veggie-rich sauces, no less, but still, 1/2 pound of pasta! i now eat my veggie-rich sauces with zucchini, and i am satisfied and slimmer. it doesn't have to be forever, but it makes no sense to eat the foods you overeat when you are on a weight loss plan.
oh, that's another thing- PLAN. have your healthy meals and snacks easily at hand. at all times. don't let yourself get hungry without a plan-approved meal or snack close at hand! and try to have something to satisfy your particular cravings- creamy (low fat yogurt, especially the greek kind, which is extra creamy, low cal, high protein, delicious-can you tell this is MY favorite snack?), crunchy (small handful of nuts, raw veggies, apples, etc) sweet (fresh fruit), tangy (pickles, etc.)
Just because you slipped up at oncel, does not mean you need to sabotage yourself for the day/week. EACH time you put something into your mouth, you have a choice.
Commit to it. Whether you want to do it or not. Whether you want to sit your butt on the couch and forgo your exercise or not. Whether you WANT to stuff your face or not. Commit to your plan. Day in and day out. Because the excitement might wane. You might get bored... or have a plateau. But commit to it. Just do it.
Know thyself. Work with yourself, start with your reality.
If you are a planner, plan. If you don't deal with failure well, set easy to reach goals. If you aren't a morning person, don't try to exercise in the morning. If you like change, change your routine as often as possible. If you are an extrovert, exercise with other people and join a weightloss group. Allow yourself to fail and constantly improve. This is a journey, but you get to choose the route.