My dd is now 15, and this has always been an issue. But while we have never denied how pretty she is, actually we acknowledge it quite freely. She is very striking, no way around it. But we have also always emphasized, that while it is nice to be pretty on the outside, being pretty on the inside is what really counts.
We have always used the old saying, "Handsome is, as handsome does".
What really used to annoy me to no end, was when ADULTS would compliment her tell her how pretty she was. But say nothing to any other children she was with. I had the same problem with my boys. I always pointed out, how this probably hurt the other children's feelings, and how feelings are much more important than looks.
What also helped us, was an acquaintances daughter. This child is one of the most obnoxious, bratty children, I have ever met. So one day I asked my daughter if she thought this girl was pretty? She immediately said no way!! Then I made her think about this child's looks, and if she had just met her, would she think she was pretty. She grudgingly admitted that yes, she would consider her pretty. I agreed, this girl is adorable! So I pointed out that even though she is pretty, it was her personality that made her not so pretty. I then pointed out one of her friends that isn't quite so pretty, and asked if she thought she was pretty. She said yes! So I pointed out that even if that girl isn't so conventionally pretty, my dd thought was pretty because it came from within. I asked her to tell me one nice person who wasn't pretty, she couldn't. It was a very good lesson, that she really took to heart. Showing that alot of how you look comes from within. No matter how pretty you are on the outside, if you aren't pretty on the inside, it doesn't mean a thing. HTH