This is my top list of things I see women do that cause them to lose their periods for a long time. In working with clients and having gone through this myself, it's a nasty process, but it can be reversed:
1. Keep track of everything you eat and know exactely how many calories go into and out (exercise...) of your body each day.
2. Only eat a certain group of foods and become stressed if you can't eat what's on your list of accetable foods.
3. Lose body weight quickly.
4. Exercise for more than an hour everyday and become upset if you miss any workout.
5. Always look in the mirror and think you're fat and hideous.
6. Never go to parties in case there's "bad" food there.
7. Never eat out, or if you do, know exactely how many calories are in your food and then exercise it off later.
8. Think your body has to look like a supermodel's body.
9. Have a stressful job that makes you feel anxious all the time.
10. Don't enjoy relaxing with people you love.
11. Weigh yourself every day and become upset if the number does not drop every day or week.
12. Spend a lot of time on the internet reading diet articles and the journals of a figure competitor.
13. Spend a lot of time alone.
14. Eat only 100 calories over your RMR.
15. Avoid dietary fat or really limit it in your food intake.