I just found this cute and handy flowchart that breaks down exactly when to address people with the formal French vous and when the informal tu is more appropriate.
It all comes down to the little box in the bottom right corner, I think. And yet, tu is always what pops out of my mouth first, because I guess I’m just a little punk.
[Photo: Marco Nunes]
talk foreign to me 
I had a complicated situation. I was an exchange student in France during high school. How do I refer to my host parents? Are they more like unknown adults or more like familiar adults? I live with them! I tried to follow the lead of my host sisters at the time, but the analogy didn’t work. Then I met the mother of my host father. What do I call her? I followed the kids’ lead, I think, which was a bad idea. My host mother used vous with her mother-in-law; it would have been good to follow her lead.
My relatives in Switzerland tried to explain the process for Swiss German. The tough thing there is that it varies by dialect–which can vary significantly only 100 km away. We native English speakers have it tough!