One thing I found that helped me a lot when I lost my weight (I was overweight in high school- have since lost around 40 lbs, and even through two pregnancies, I've managed to keep it off), is replacing your favourite foods with new favourite foods.
I replaced things like brownies and cakes (my vice >.<) with things like unsweetened yogurt with honey on top, and hommeade apple crumble. I replaced hot chocolate with teas. And yeah, maybe the taste wasn't as comparable, but I felt good eating those foods, because I knew that I was treating myself AND doing something good for myself, and I found that that feeling- being proud of myself- really motivated me to keep it up. Six years later, and I'm still nomming yogurt as a favourite snack.
And exercise is invaluable, because not only does it help you lose weight in the first place, it will help you to keep it off later. The more active you are, the faster your metabolism runs, the easier you burn through calories. Plus, it'll add an extra incentive to eat healthy, because you'll notice once you start being really active, you feel great when you're well fueled, and you feel crappy when you're not eating right. It really helps.