My version of Chocolate Cake's examples:
Her: "Your hair is flat and lifeless"
You: "How kind of you to say."
Her: "What's happening to your waistline?"
You: "How kind of you to notice."
Her: "You're getting crow's feet"
You: "How kind of you to say."
Her: 'I see you're picking at your fingernails again."
You: "How kind of you to notice."
ALTERNATELY:
Her: "What's happening to your waistline?"
You: "Yeah, I'm putting on weight to see how many people I can get to make rude comments to me."
Her: 'I see you're picking at your fingernails again."
You: "Yeah, I did it to see how many people I can get to make rude comments to me."
If you want to, you can turn around these comments in your mind to make them something positive for yourself:
You get a point/star/tick mark/check for every criticism she lobs at you. Once you get to, say, 10, you get to buy yourself a magazine/tube of lipstick/bottle of nail polish. Or you can make it a larger goal, like for every 50 you get a pair of shoes or a manicure or something.
I did this once when there was a woman I found annoying in a meeting I was in. I spent the morning with my head about to explode, went to lunch, came up with this system, and WANTED her to do the annoying thing in the afternoon session. I got a star every time she did it, 10 stars = 1 bottle of nail polish. I got THREE bottles of nail polish out of that afternoon.

You can't necessarily change your friend's behavior, but you CAN change how it affects you.