DS had a girl in his class in Canberra who home schooled 3 days a week and did 2 days a week of Italian immersion. To do that obviously you need to be near a bilingual school that is prepared to accommodate you but I'm sure there are other ways of harnessing outside help.
An au-pair or nanny who uses the language, there are lots of language playgroups, we listen to language CDs in the car.