I would slightly disagree with the answers previously given - well, more than slightly. They either do not distinguish between "ogling" and "admiring" or encourage us to communicate to our kids, implicitly, by non-action, that it is ok for other people to disrespect them. Ogling is defined as "staring in a lecherous manner", with lecherous defined as "having or showing excessive or offensive sexual desire." If I saw anyone "ogling" one my girls, in my presence, I would respond by moving to stand between my girl and them, and very pointedly staring down the idiot.
In fact, I have done this on three occasions. Once with a man very obviously staring at my wife's butt, once with a teenage boy staring at my oldest daughter, and one other time when I chaperoned a group of girls, including my oldest daughter, to a volleyball tournament in Papua New Guinea. When we arrived at the place and were disembarking from our van, a group of local men started staring, pointing at our girls, snickering and making rude comments. I stepped between the girls and them, gave them the evil eye, and curled my mouth in my best look of utter contempt.
In every one of these cases, the ...[insert unkind adjective describing lack of intelligence, stature and/or basic hygiene]... men in question quickly became embarrassed and turned away.
The thing is that you are educating your kid when you do this. You communicate to her that she is valuable as more than as an object of "excessive or offensive sexual desire." You are setting an example for what she can do when you are not around. I would also encourage you to talk to your girls and let them know that when a boy or man looks at them in an "ogling" way, it is perfectly ok for them to expect and demand respect. More often than not, a good stare is all it takes.
But, of course, you also need to discuss these things with your daughter. Discuss what happened, if it happens, and why you dealt with it like you did. Sometimes, I suppose, it is ok to ignore the ogler, but she needs to know that she can, if she feels comfortable and safe, expect and demand respect. She can communicate that with a good stare, but she can also choose to walk away.
It's another thing if a man or boy looks at your girl in a way that says "wow, you are beautiful." I have had that too. I have three girls and a very beautiful wife. I had a coworker comment on my wife's beauty when he first met her. It was clearly not a lecherous comment at all, but a respectful admiration. I thanked him with a smile from ear to ear. I also had a guy ask me if he could take my daughter to prom. Every time he looked at her, I could see this look of total admiration in a non-lecherous way. I could tell he respected her. I knew the request was coming and was more than happy to give my blessing.
By doing these things, you also set a pattern and you are training your boy(s) if you have any. I have an 18 year old, and he has learned to respect girls around him. I have talked with him many times about how to treat girls.
It's ok to admire. It is not ok to ogle. If our every word and action communicates that to all around us, our girls, our boys, the idiot at the truck stop during a long road trip, then maybe we build a culture that can admire female beauty while respecting the humans who bear it.