As a start, you should always project the opposite of what you're actually going to do. If you're going to cut yourself, smile! People will tell you you're the happiest girl they know. If you're going to starve yourself, have desserts. You'll have your mom telling you to stop eating so much. The great thing about people is that, once they see you a certain way, almost nothing will change that.
You can't get lazy though. Always take care of the evidence. I don't mean hide it, because hidden things can still be found. If you need to hide evidence, you've already been too careless. Taking care of the evidence is part of the plan. You can cut your arms, but only in winter, when its normal to wear long sleeves. Switch to your inner thighs for summer, and take care the swimsuits you choose. You'll have to eat dinner with the family, but you can throw away most of your lunch during the school day without anyone noticing.
IF, and I do mean if, someone asks questions, don't panic. Trust that your precautions have protected you. The only thing panic can do is make you defensive and getting defensive is the surest way to get caught. Surprise is better. "Really? Me lose weight? I don't think so. I hadn't noticed it."
The fewer words you say the better. You don't want them hearing something in your voice. Keep your focus on whatever you were doing before, extra points if its your phone. Sometimes all you need is a disinterested shrug for the whole conversation to go away.
IF you get someone who is unusually persistent, still don't panic. Turn to half-truths instead. They sound plausible, and people eat them up. "Oh I would never do anything bad to lose weight. I guess I might've lost a little because we're in season right now." If you're feeling a little irony, thank them for their concern. It'll make them feel good about themselves and the whole thing in general. I doubt you'll ever hear another question from them again.
Just remember this: there is a trade off for everything you do. You have chosen, in order to get away with whatever you're doing, to live the life of a liar. You have sacrificed your innocence, your fun and your conscience to control the truth. The life of a liar is no easy way to live.
*Do you regret it?
I don't condone the use of these tips to harm anyone, except apparently myself