Three of my siblings are bi-lingual Russian/English. At first they mixed things up terribly, but after their Russian grandmother spent a year with them they were suddenly able to completely separate the two. Grandma doesn't speak a word of English, whereas both parents speak both languages, so we speculated that what they were doing was 'the easiest thing that works'.
When I speak in Norwegian I toss in English words all the time (because I can get away with it - all Norwegians seem to speak English). In French I don't leak English into it. In English, I rarely (if ever) slip Norwegian or French into it.
Where I really get into trouble is when I'm in a conversation with Norwegians, French, and English-speakers at the same time, because I end up trying to translate Norwegian to French, French to English, etc, and end up speaking the wrong language to pretty much everyone involved.