It is important to recognize disordered eating patterns such as:
-fasting for long periods of time
-bingeing and purging
Setting healthy examples is very important. Parents and peers should encourage developing healthy eating and exercise habits that do not border on extreme dieting. It is also important to show and tell that everyone should be accepted and loved at any weight. It is also important to give praise for characteristics other than appearance such as intelligence, compassion, self respect, and generosity.
Take a look at this blog post that talks about how a girl views body image.