Since literally the minute my kids were born, I speak only French to them. I have my reasons; feel free to ask!
Here’s how I did it:
I’m fluent because I majored in French, and spent a year of study in Paris. And I knew about language acquisition through my graduate studies.
We use the One Person, One Language method: I speak French when I speak to each or both kids. My husband speaks English to them; I speak English with him. (The By Place method also works: for example, English at school, Chinese at home; or English at home, Hebrew at temple.)
That’s the basics of it: consistently use the same method. Below are extras that helped.
Note: I had to take a guess, and decided to let my kids respond in the language of their choice. That’s usually English, unless they’re trying to butter me up. They don’t have French-speaking grandparents, so I felt it best for our situation. I’ve seen families force kids to reply in French, and sometimes they develop a resistance to the language; my kids never did. But when they are with French-speaking people, they speak French, of their own choosing.
We had French-speaking au pairs for a few years. We still visit each other.
The kids went to a French preschool for a couple of years. Then they had a weekly French lesson, so they could learn to read and write in French.
We traveled to French-speaking places for vacation, when we could. We purchased films, books, and music in French, so we could use those materials at home.
When they were in third grade, they could start taking French or Spanish in school. So, we chose Spanish, since they had already had 8 years of French at home. Spanish was easy for them, since they already knew French.
When each kid was 16, they spent a month in a French-speaking family, without me. In high school, they took A.P. French.
Unless one or both parents are native speakers of another language, almost no one in the U.S. is doing this. I had to be a pioneer. But my education prepared me, so I was confident it’d work. And it did!