Use With Caution
While it's dangerous to overgeneralize a culture, it should be noted that the French largely do not take pick up lines very seriously. Rather, they are largely regarded as jokes. Don't expect these to work like a magic charm on native French speakers — if they do, it's probably because they find it cute that a tourist would try such a thing. Same goes with victims that aren't native French speakers. Just because you happen to be speaking a little French doesn't mean that your target won't find these sexist, antiquated or just plain silly — and they might tell you just that. Ah well — c'est la vie!
Over the Top
French has a reputation for being over the top when it comes to pick up lines, which is only slightly undeserved. These lines are tuned towards picking up those of the female gender, though they are easily modified by substituting Mademoiselle for Monsieur, le fils for la fille, etc. Here are a few popular ones, and an idiomatic as well as literal translations where applicable:
Est-ce que ton père a été un voleur? Parce qu'il a volé les étoiles du ciel pour les mettre dans tes yeux.
Was your father a thief? Because he stole the stars from the sky and put them in your eyes.
Tu n'as pas eu mal quand tu es tombé du ciel?
Did it hurt when you fell from the heavens?
Est-ce que tu crois au coup de foudre au premier regard ou est-ce que je dois repasser?
Do you believe in love at first sight, or should I pass by again?
Est-ce que tu as un plan? Je me suis perdu dans tes yeux.
Do you have a map? I have lost myself in your eyes.
Comment dit on “I love you” en francais?
How does one say “I love you” in French? (Feel free to reply, Je t'aime aussi, I love you too, when they tell the answer!)
On the Subtle Side
These, while quite stereotypically French, may be a little over the top for your style. Here are a few pick up lines that don't quite grate as much on the cheesy side — ones that seem more like conversation starters than anything else. These are probably better suited for native French speakers.
Salut! Tu viens souvent ici?
Hi! Do you come here often?
Pardonnez-moi, mais ca a l'air delicieux! C'est quoi?
Excuse me, but that looks delicious? What is it?
Best suited for a cafe or restaurant setting, obviously.
Je peux vous inviter a boire un cafe?
May I buy you coffee?
Good way to get someone into a situation where you can sit down and charm them a bit more thoroughly. It's hard to resist a free coffee!
Excusez-moi de vous deranger. Comment s'appelle ce quartier?
Excuse me for interrupting you. What is the name of this neighborhood?
Good for playing the hapless tourist card when traveling abroad — showing an interest in your surroundings. With any luck, the person can be coaxed into giving you a personal tour of the place, which should be interesting even if nothing comes of it.
Make Your Own
There really isn't anything special about these: imagine something suitably saccharine in English, and translate it. You might want to run your concoction by a native speaker before doing using it, however, as there are many tricky idioms and false cognates that you might want to avoid! The Internet is a great resource for this. Many websites such as WordReference.com host forums where you can ask these sorts of things to native speakers. This is certainly a popular topic, so you'll probably be able to find ideas from other people's queries as well.
Bright Hub has many articles on learning French, some of which may be of particular use for constructing your own pick up lines:
Lesson Plan Idea
Puzzling over how to teach this to your students? Here's an idea: Try pairing up your students into groups of two and try having them create their own pick up lines. Make it into a competition as to who can come up with the most outrageous (and grammatically correct) pick up lines. Or, have them come up with a skit in which they face off — perhaps one resistant to the other's advances, or one playing a hapless tourist who slaughters the idioms with the other person corrects them. Encourage your students to be creative! This is an excellent way to have them engage idiomatic French structures, and will also be tremendous fun for students once they get over the initial awkwardness.
AskMen.com: Top 10: French Pickup Lines, https://www.askmen.com/top_10/dating/top-10-french-pickup-lines.html