Very few things scare me more than my WEEKLY visit to the doctor; she's way too damn smart to let me get away with any of stuff I'm about to tell you to do. These tips pretty much only work if your doc doesn't know about your eating disorder.

• Drink a lot of water before you go. A pint of water (16 oz) is about a pound. Note: This won't work well for you if they do a urinalysis (make you pee in a cup); your doc will totally know you've been water-loading.
• Practice meditation to control your heartrate. It may sound stupid, but this saved my ass a few times. A side-effect of starvation is that your heart doesn't hold up very well when you go from lying to sitting to standing; your pulse tends to skyrocket. Focus your mind on slowing it down.
• Blood pressure is a tricky thing. Like your heartrate, your BP is negatively affected by starvation. Put a little salt (about a teaspoon per glass, more if I'm in a hurry and my stomach's not upset) in each of my glasses water (not too much, 'cause it'll shock your system and make you retch) or I'll eat some celery with salt on it. Salt raises your blood pressure for a short time and can help fool the doctor. Downside, it makes you retain water, but if you drink alot of regular water after your appointment, you can get rid of most of that. If you're worried about the effects wearing off by the time you see your doctor, get a Nalgen, or other dark colored or opaque water bottle and bring it with you. (Millions of thanks to Chloe for submitting this info!)
• Dress warmly; even if they make you put on a paper gown, your body will hopefully retain enough heat that your temperature will be somewhat normal.
• Do anything to make yourself weigh more: drink water (see above), wear big shoes or a giant belt, put change in your pockets, anything. Ladies, some coins in your bra add weight, and you won't have to take off your undies for weighing!
• If your throat is sore from purging, make an obvious fuss about it; say you've been waking up with a sore throat for a week now. That way, when your doc checks, he/she won't be surprised to see redness and irritation.
• Lanugo is a dead give-away. The razor is your friend.
• Speaking of razors. Cuts, burns, scratches, bruises...anything you do to harm yourself...it will ring alarm bells with any doctor. You know your body and you know how long it will take to heal. You also know when your doctor's appointment is (hopefully). Make sure you're all healed up before you go see your doc, and if he/she notices any scars, say you've stopped and you don't plan on starting. I know how hard it is to "just not cut," but trust me, it's better than the alternative.
• Your doctor is held to confidentiality. Anything you say during your appointment cannot be repeated unless you say so. So if your doc isn't fooled and confronts you about your eating disorder, tell him/her you want this to be kept confidential. This is especially important if you're under 18, since some doctors will go ahead and tell your parents anyway...UNLESS YOU SAY OTHERWISE. Don't try to deny it; you'll just make things worse.