While I was in Japan, I noticed a few things.
When I stayed in a hotel, and it had CNNJ (which is mostly in English if you want it) or the BBC, the more I listened to it, the
worse my Japanese became.
If the hotel (usually tiny business hotels, because they're cheap) only had JP TV, then my Japanese either stayed at level or improved a little.
I'm thinking that a lot of that is due to just being surrounded by it constantly, and more importantly,
not letting yourself slip too easily into English mode. As the trip went on, I tried to avoid using English as much as possible, and it helped.
The other thing I noticed, which amused me to no end, was that when I got back to the States, I would sometimes say "どうもありがとうござました" (really fast) to store clerks when I finished paying. I even caught myself saying "すみません~" to someone I almost bumped into at the parking lot.

It really does become a habit after a while.
My background in languages is about 5-6 years of French in school (which I promptly forgot, except to confuse it with the German I have trouble remembering), 1 semester of Russian in HS (of which I only remember how to say "I don't speak Russian" and recognize cyrillic), 4 semesters of German in undergrad (which still makes my mom's side of the family in Germany cry-- I really should study more German sometime), and then after a long break, 6 semesters of Japanese in grad school and beyond. Japanese is SO different, and so difficult, that I'm becoming obsessed with it. XD
But even though my mom is German, I didn't grow up in a bilingual house. Sometimes I think she should have just spoken German to me all the time anyway, but I think she was always trying to improve her English.