Dude, this is simple actually.
I used to work at a bar and did this.
Firstly, get a nickname for the girl. If she's
wearing an orange shirt for instance just say "and here's the salad for
orange" as you deliver her food. In the end, after she is comfortable with you and your playfulness
just get an extra bill (receipt) and before she goes say: "Miss, you forgot your bill" and smiling give it to her. Inside that bill you will have artfully written something along the lines
"Hey
Orange. I wanted to say this personally but your dad/mom/brother looks so mean I bet he would have killed me with that chopstick of his! (assuming it's a chinese restaurant).
So here's my number. XXXX-XXXX. Text me. If you don't Im gonna be
sad

" (Draw a smiley face after you write "sad". It's push-pull on paper!
And if she doesn't text, don't worry. The next time she comes to the restaurant just look at her and say "You're not used to getting hit by handsome bartenders, are you?" and smile. Smile smile smile baby!
It's all about having fun.
Good vibes!
=]