I love being approached by people who want to talk, and I love approaching people to talk. I make friends on the subway, and talk with some of my customers beyond selling them things. I like the first adrenaline rush when you approach someone for the first time. It's fun to see if people are going to reach back or not.
The best approach seems to be to mention something in the immediate area and comment on it, like the previous posters have mentioned. Make sure you smile and are friendly, without being too hyper or sleazy. If she smiles back and the conversation goes well, feel free to casually mention that she's pretty.
Don't use the words "hot" or "sexy" or whatever, but a nice "I think you're pretty" can make her day. Or at least, it can make my day.
At worst, it gets awkward, but you never have to see that person again. Most people won't be rude, and if they are, then they weren't worth knowing.
Take the risk and talk to her. It'll be worth it in one way or another.