You will need a smartphone with photoshop and internet.
Walk up to a girl and give her the classic; "Don't I know you from somewhere?"
If you are lucky she responds friendly and baits.
If she doesn't and gives you the "erhm, no." Look at her like you are really baffled by how familiar she looks to you. Act unconvinced and ask her for her name. She might only say her first name then try luring out her last name as well for this will make the next part a whole lot easier. When she has given you her name, ponder it for a while (absolutely take your time to remember it) and then say something like "Hmm... it sounds really familiar... O well" and walk away.
Now go on Facebook and search for her. Hopefully she has photo's off her group vacations/travals/whatever. If not, try checking her friends photo's, repeat till you found some. After you found a photo, photoshop yourself in the background. Take your time and try to get as much info as possible (comments!).
Now with the photoshopped photo, walk up to the same girl again. And say something like: "Here!! I knew it! We met on that one holiday!"
If you are convincing enough she well be a bit embarrassed for not remembering you. Which in her mind made her look like a total bitch. She will feel obliged to spend time with you to make up for it.
And you are SO in!
For more information check out my official website!