When to Go: Winter, for intercontinental couples. The French are very sensitive to cold weather and the snow makes them domestic and (somewhat) faithful. For single travelers, try visiting in the Fall. Spring and Summer are overrun with tourists and you don't want to be mistaken for one.
Where to Stay: Definitely not in a car or on the street, unless you want to be mistaken for a gypsy and deported to Romania. French men are pretty open, sociable and patrons of the arts. If you just say you're "visiting on an artist's visa," it implies that you're good in bed (bon au lit) and you'll score one for free... even if you have no intention of sleeping in it (see Sexual Health section).
Language: French, but speak it only if you're conversationally fluent (frogs have no patience for beginners no matter how cute you are). Many people also speak English and if you ask politely (parlez-vous Anglais?) you won't come off as presumptuous. If being insulting is more your speed, speak to them in Spanish.
Slang: "Du ju haf e boyfrie(n)d ?" means 1. Are you interested in sleeping with me? 2. Would your boyfriend mind if you slept with me? 3. In a fight, could I beat up your boyfriend?
What to Eat: Nothing (that's the real secret to why French women don't get fat).
What to Drink: Red wine, the only item on a menu that's cheaper than water.
Getting Around: Paris is walkable, bike friendly and the metro is expedient and timely (but closes at 1 a.m. during the week and at 2 a.m. on the weekends). Taxis are another option and pretty easy to track down, unless it's past midnight (you'd have better luck convincing a NYC cabbie to drive you to Brooklyn on New Years eve). Your best bet for late night transportation (and again, exercise caution, or at least don't sue me) is walking into a night club, flirting with a drunk Turkish man (they consider eye contact flirting) and getting his sober friend to drive you home.
Sexual Health: With chemical castration of sex offenders legalized in 2004, the rate of sex crimes in Paris has significantly decreased, but since you may be shacking up with strangers to avoid hotel fees, it's important to exercise caution when choosing platonic bedfellows. For more information (once it's too late), call the rape crisis hotline (Viols Femmes Informations; 0 800 05 95 95; 10am-7pm Mon-Fri) and hope they speak English.
Useful Phrases: As tu un préservatif ? (Do you have a condom?) ... non, vraiment, je rigole pas (No, really, I mean it.)
Best HIV/AIDS Test: Centre Médico-Social (218, rue de Belleville) call for hours - 01 40 33 52 00 ... I've never been, but Belleville is an artsy hipster/immigrant neighborhood and a good place to walk around and grab a cup of coffee (maybe your last, jk) while awaiting your results.
Safety Tip: If you're shopping at the Publicis drug/bookstore on the Champs-Élysées, be aware of asshole security guards who may chase you out of the store and yell at you in French for treating it like a Barnes & Noble (i.e., reading a book with no intention of buying it and then not putting it back where you found it).
Bizarre Fun Fact: Roman Polaski, acclaimed director and Swedish house arrest ex-pat, is said to be currently residing in Paris, as is American comedic author David Sedaris (did someone say ménage à trois?)