Learn at least a little of the language
These four basic words will suffice: "bonjour" (hello), "au revoir" (goodbye), "s’il vous plait" (please), and "merci" (thank you). Immediately speak English with a French person and they will pretend to not speak English. Speak French with a French person, they will switch into English. It’s just how it works.
Pleasantries are mandatory
Say “bonjour” and “au revoir” to every shop owner, salesman, and server every time you enter and exit a store or restaurant.
Step up your clothing, but don’t worry too much
Popular opinion will have you believe that all Parisians are beautiful, svelte snobs who only wear designer clothes. Which is mostly true, but France has loosened up dress codes considerably in the last 30 years. Plenty of people wear sweatshirts, shorts, and tank tops. You can too!
Try not to be... so American
Keep your voice down if you want to blend in. The French tend to mumble inaudibly.
Bread goes on the table, not on your plate, and you better not butter it
Unless it’s breakfast, butter likely won’t even be an option. But buttered bread with shellfish is totally okay, for some nonsensical reason.