What I do is Slightly rub her back,(breaks physical barrier) but this all happens in a split second. I also look them in the eyes, and I show confidence, and I talk alot. There's nothing worse than standing there all silent staring at each other.
What people don't realize is that there are so many things to talk about, usually because we are nervous we want to think about what we say, but there is no time for that, just blurt out whatever, "I was at the supermarket last night and I saw the strangest fruit ever." see it doesn't have to be meaningful or personal, that right there is flirting, if she's interested it doesn't even matter what you say as long as you don't make it awkward.
When I compliment them I usually compliment them on something special, not cheesy or obvious. Ex:
"Hi I don't mean to sound weird or anything, but I just noticed you have the prettiest eyes I have ever seen"
too good things can happen
1) She will remember you for this comment and she will feel confident and more interested in you
2) If it didn't work out at the end, at least you made her day, and she can go home happy.
Finally if your going for the number
Let her know that if she chooses not to take you up on your offer, you'll never bother her again
so she doesn't feel pressured or chased.
give her a chance to think back on what happened and make her wonder "hm.. I wonder how he's like? I'd like to get to know him more."
This is just the tip of the ice. You can do anything and the possibilities are endless. I know it works and yes it sound easy because it is.