I have had great success complimenting a woman on her looks as part of opening her during a completely cold approach. It happens in this sort of context: It is day time, on the way to work or during lunch, she is dressed great and really well put together--so it's in context. My comment has been something like: "I appreciate what you got going on there...You must have had something on your mind when you got ready this morning..."
I leave it hanging like that and wait for her to fill in the blanks. If she replies that she's not sure what I mean, I answer by saying that most people don't put that much thought into their appearance and that I appreciate her efforts. I then close by saying I hope she doesn't let it go to waste, and suggest we catch a happy hour drink that evening.
If delivered with confidence, it works really well. Even if she can't make it that evening, you can give her your number and tell her to drop you a line the next time she's dressed great and ready to go.