1. Buying low-fat versions of everything. They usually add more salt and sugar to make them taste good, so it doesn't really help that much.
2. Only doing cardio, and not lifting weights. Adding muscle is what kick-starts your metabolism. So you should really do both.
3. Not getting enough sleep. You're more likely to gain weight if you get less than six hours a night. Seven or eight hours is ideal.
4. Setting unrealistic goals. You have to think long term, and not expect to lose a ton of weight overnight. Otherwise you'll get discouraged.
5. Not drinking enough water. It makes you feel full, so you're less likely to snack. You're also less likely to drink stuff like soda and juice that have a lot of empty calories.
Source: Women's Health