- Love yourself enough to believe you deserve to be thinner.
- Weigh yourself every day, especially when you've been bad.
- Three times a day, say aloud why you want to lose weight: your health, looks, career, your kids, whatever. Say it with expression. That will brand it into your brain.
- Most important: Be conscious of every bite, deciding whether the momentary pleasure of that bite outweighs the liabilities.
- Eat something small whenever you're slightly hungry. Stop when you're not hungry, not when you're full.
- All good diets reduce to eating lots of veggies, some fruit, a little protein, a little whole-grain carbs, and very little fat.
- Eat the same things most days: the low-calorie but filling foods you most like and are easy to prepare.
- When you screw up, forgive yourself and get back to your diet.
- Find rewards other than food.
Update: A commenter asked why I didn't mention exercise. That's because current research shows that exercise, while healthy for other reasons, rarely helps you lose weight. Why? Because appetite increases in proportion to the amount of exercise you do. When ravenous, it's hard to eat little. Worse, having exercised, you feel entitled to eat more. HERE is a review of the research literature on the topic. And HERE is another article on the subject, which both discusses the literature and links to a Time article that also concludes that exercise is unlikely to help you lose weight.
It's all about the calories--but that wouldn't sell gym memberships.