I try to avoid commenting on my DD's appearance as much as possible. Really, I just want to tell her how breathtakingly beautiful she is all the time, but I try not to do it too much (or at least only as often as I tell my DS he is beautiful). I try to comment more on how interesting and funny she is, and what a kind and thoughtful friend she is.
I have also had some quite mature chats with her about how 'prettiness' is just luck, and not something that requires any effort or skill from the individual. People who happen to be pretty are just lucky, whereas being thoughtful and considerate and working hard are things that anyone is capable of, and which require effort.
I've also told her about airbrushing in magazines and adverts, and popstars who have oodles of help from stylists and make-up artists. I've tried to help her understand that very little of it is 'real'.
We've talked about how prettiness is something that fades when people get older, and that when we are all very old, it really won't matter how pretty we once were. What will matter is the lives we've led; the things we've done, the places we've been, the people we've loved.
I hope you find some ways to help your DD.