J.M.
From experience the only successful way I've found is full immersion in the language and culture. My first born was in school in Mexico from KG to the 6th grade. She is about 90% fluent in reading, writing and speaking. My 2nd born hated the school in Mexico and even though she stayed till 3rd she is only 20% fluent. My twins who are 5 y.o. had 2 full-time Spanish speaking only Nannies and can barely speak the language. They understand for the most part in basic Spanish questions but only respond in English. I believe it is because my husband and I speak English in the home. I home schooled my dtr in English and found sending her to a Spanish school is the only thing that has been successful in our home. I have 5 children and the first born is the only one that is able to speak well in Spanish.