I tested this only 2 times, but it was working.
The ideal type of target is a single moving girl that is walking slowly (she does not seem in a hurry).
Go to walk next to her at her same pace for a few seconds (something like 10 seconds). Then say something along the lines of: "Did you notice?". You will get her attention. You: "We are walking side by side like it was a date" (cheeky face). If she laughs, you got her.
Then you can continue how you want. I tried something like: "You know, a flash-forward! Like in the books or the movies. We are here together, the readers or the viewers still do not know why but etc..." Or "Since it seems a date, we could continue to simulate it and do the usual stuff etc..." (you basically try an instant date).
The first girl that I tried this on had a boyfriend, but she got my sense of humour. She was going to have dinner and she invited me with her. I refused because I had the feeling that she was just being social and she wanted some company (I should have gone anyway and flirt).
The second one also appreciated the idea and laughed all the time, but she rejected me in the end.
Try it!
If the girl does not play along, you can always come back to the old strategy. Go in front of her and say: "Wait! I was joking, but I find you very attractive and blabla".