How does a child become bilingual?

This is basically a response reading 2.1, part 2.2 How does a child become bilingual?
The initial answer that was proposed in the reading was by growing up in a bilingual environment.
Being a parent and living abroad, I initially thought that creating a very supportive home environment in one language, can coexist with a different outside environment. When my daughter was born, I imagined creating such a supportive English language environment at home (yet outside the home everything is Japanese), follow the one-person one language maxim, and instill within my daughter a love for both languages, so much so that she would be proud to speak both languages in all situations. Alas, my hopes and expectations were somewhat too idealistic and definitely did not account for all possible affecting variables (psychological and social). Don't get me wrong, I tried my hardest to make those dreams a reality, but if I knew then, what I know now...alas regret, something I try to avoid...
According to the reading ~ exposure, consistency, perceived need, and social support all factor into the mix, however, I feel that the social aspect has a considerable impact.
1. Providing a chance for exposure to take place is sometimes difficult to create. When my daughter was young, it seemed easier, with videos and books, but as she grew older, and especially at the time she began school (regular school), it was harder and harder to create opportunities for her to use and be surrounded by English.
2. Consistency (in staying in English mode, or responding in the same way each time) is difficult to maintain, especially when I am also trying to learn the language of the society I am living in. I find it hard to only respond in English for at least two reasons. One is that I want to use the language I'm learning, and the other is that I want her to be able to really understand what I am talking about. Her Japanese seemed to suddenly take off at around age 5 or 6 and English kinda lagged behind... Sometimes it was easier and faster to speak in Japanese. I also often find myself talking in mixed English and Japanese, using the words that I feel more comfortable with, and that would get the job done.
3. When my daughter started school, her perceived need to speak English suddenly dropped, because she wasn't encountering English that often (I was the only person who spoke to her in English). It wasn't that she didn't understand English, it's just that she had few opportunities to speak it... and slowly but surely, she started reverting back to Japanese, even when speaking with me... My choice was either to encourage her to speak only English, or let her speak Japanese and speak to her in English. In the beginning, I chose the first one, but it didn't work...I eventually moved on to the second option...But now there is nothing motivating her to speak English...
4.It seems sometimes, that all her outside influences are just too strong... That and my wife only speaking Japanese to her...
I am happy with the English she does know. I feel that when she was young, I exposed her to enough English, for a fairly long time, in that so called critical period, that English will stay with her, in some way. She may always have that feeling for the language. My worry now is with her language attrition (loss of English), but I believe that I can support her now and in the future. The kind of support varies as time passes, and she goes through different stages. But I discovered, it's undesirable if you push too much and it's also undesirable if you don't expose your child at all. It's really a fine line...
Somewhat personal today
ciao
Steve