Consider rewards or penalties. It sounds a little harsh, but give it a second. Let's say you set up an "English only" rule at the dinner table at home (a really good idea, by the way); how long will it last? Probably not very long. But if you have some type of incentive program (if we speak only English for two weeks straight, we'll go out to eat, etc.) or punishment ($1 for every time the native language gets spoken, say), it'll be a lot more likely to stick.