I'd like to pitch the identity angle, which worked for me very well (your mileage may vary, of course). I ate very little processed sugar foods (chocolate, cookies, etc) at various points in my life due to what I saw myself as:

• "I'm not the kind of person who eats processed foods, because processed foods are yucky" is part of our family lore and works to this day
• "I'm not the kind of person to waste money on things like cookies if I can make them myself for less" arose during a low-income but savvy time. It works because when you bake cookies/cakes yourself you're free to use much less sugar.
• "I'm the kind of person who enjoys simple foods/Sugary foods are an indulgence, so of course I don't eat a lot of them", my current one.
• "I can get addicted to substances very easily, so better not overdo it" (also works for alcohol)
• "I'm the kind of person who doesn't snack between meals, because it's uncultured" - trying to incorporate into my identity at the moment.

Once something is part of your identity, following it becomes a joyful, self-affirming activity rather than a willpower drain.

I also found that when I'm eating common supermarket sweets, I eat a lot because I try to satisfy a craving for flavour that these foods lack. If I substitute them with home-baked flavourful cakes or good chocolate, I tend to eat much less, since my craving is satisfied with the first bite. I'm not sure how making a calorie-rich food (that you don't eat a lot of) flavourful influences your body fat setpoint though (http://wholehealthsource.blogspot.jp/2011/04/food-reward-dominant-factor-in-obesity.html).