Wear baggy clothing and lots of layers. This will hide weight loss and help keep you warm.

Pack a a bag lunch to bring school, and make sure people see you make it. Then, when you get to school, throw it out or give it to someone else.

Hide diet pills in your piggy bank. I did this for months and no one ever knew. Rotate where you hide them, too, just in case any gets suspicious.

If you keep an ana or food journal, carry it with you at all times. Never leave it lying around for other people to find.

Another journal tip: don't write in your language, or don't use the regular alphabet. I like the Phoenician alphabet myself, and Elvish looks really cool. The Omniglot site has tons of alphabets you can use.

Purge in the shower, or turn on loud music to cover up the sound.

Clear your internet history after viewing pro-ana sites. Tools > Internet Options > Clear History.

The Opaque Cup Trick (spitting out food into an opaque cup instead of eating it) is very risky and should only be done in emergencies. Better to spit it out into a napkin; that way, if caught, you can say you bit into something gross.

Putting band-aids over your knuckles when purging will help prevent scarring, which is a major indicator of an eating disorder.

If you know that you're going to purge (with laxatives or through vomiting), complain loudly that you feel sick. Make a big show if it. That way, if someone catches you, they won't be suspicious. Obviously this won't work if you do it constantly, though.

Using a nail-growth nailpolish (such as Nailtiques) will keep your nails from becoming brittle, which is an indication of malnutrition.

This may seem obvious, but it's difficult: don't constantly talk about food and/or your body. First off, people don't like it. Secondly, obsession and preoccupation with food and your body is sure to tip someone off that you have a problem.

Leave dirty dishes and food wrappers lying around the house; that way, people will think you've been eating. Do this enough and you may even be able to use it as an excuse to skip meals.