I get asked a lot of times by all kinds of guys:
"whats the BEST pick up line in THAT situation?"
"I was at a bar and this beautiful young lady started starring at me.. so 5 MINUTES later I approached her and said "Hey You are really beautiful. I bet You have a boyfriend"."
Now do you see a mistake here?
These men really are acting like wussies. Having success with women is about being sure of yourself and not having doubts of what you should say in each situation.
You need to have a strong feeling inside of you that you are THE MAN, not just another hopeless frustrated single guy. Saying "hey sweetie you are gorgeous" will just make her think you are like EVERY other guy who approached her that day. You need to show her you are different, that you are used to approaching women and having success.
By saying "I bet you have a boyfriend" you are assuming she is committed and doesn't want to talk to you. You are transmitting the wrong message.
There is a sentence David Deangelo uses to say that I think its perfect for this situation: "It's not about WHAT you say, but HOW you say it". If you can communicate you are a high value guy, almost anything would work. Even a simple "Hi".
And how can you communicate you're a guy who usually has success around women?
This is what we call the cocky+funny technique. It's some kind of magic bullet.  With it, you tease the girl in a funny way, making her feel a DEEP level attraction towards you. It's POWERFUL.

For truly understanding all the psychology behind this, and how approaching and attracting women really works, I recommend the Double Your Dating ebook, by David Deangelo. I've read it like 5 times myself and I can tell you: Everytime I read it I figure out something new that happened to me while approaching women and I couldn't understand.

Check out this Double Your Dating report that you should read right now if you are interested in getting more advanced.