1) Compliment her on something she doesn't get complimented on often.
If she's a beautiful girl, chances are she gets told that a lot. If she's musically talented, chances are she probably gets told how good she is at music. Et cetera, et cetera. If you really want to make an impression (remember: it has to be genuine), compliment her on something that she doesn't hear often. It'll mean a lot more to her. And it will also surprise her that you're not complimenting her on something obvious and/or that she's used to hearing. She'll most likely remember you as "the guy who said ____".
2) Compliment her on something NOT physical
As I'm sure you have experienced before, some girls can be really attractive, yet when they open their mouths, they somehow become less attractive. And the opposite is also true. Looks and character are two completely different things. Don't just compliment a girl on her physical appearance. What about her character is attractive? Get to know her. Find that. And compliment her on her confidence, her intelligence, her humility, her visionary mindset, whatever character quality(ies) stands out to you the most.
3) Don't compare her to celebrities...unless you KNOW she considers them beautiful.
I've had a guy attempt to compliment me by saying I looked like a certain actress and, while his intentions were sweet, I didn't consider the said celebrity very attractive so...the compliment fell kinda flat for me. Do NOT compare her beauty to an actress or model or singer unless you've heard her talk specifically about them in regards to how beautiful she thinks they are.
4) Don't ever use the word "pretty" to compliment her.
Calling a girl "pretty" is basically the equivalent to how you guys perceive "You're a good friend". It's like telling a little girl how pretty she looks in her princess costume. That's not the word you want to use for your stunning date you're taking out to a fancy restaurant. Or the confident girl who snapchats you after a work out. That's not pretty. That's _______ (fill in the blank).
5) Know the difference
Compliment: That color looks very nice on you.
Hitting on: That color is sexy on you.
Compliment: Your smile is very lovely.
Hitting on: You're hot.
Time and place, people. Time and place.
6) Compliment her without words.
"How am I supposed to do that?" you might be wanting to yell at me. But c'mon, guys. YOU of all people know that sometimes words just aren't good enough. A picture paints a thousand words and sometimes, when you look at the girl who takes your breath away and leaves you speechless, a thousand words are nowhere near your tongue. Maybe not even one word. So what do you do? You gaze at her in complete and utter adoration.
And trust me. Us girls know that look.
And we love it.