Hi! I have been searching for answers to this question for a while now. One common answer is that there needs to be human interaction with someone fluent in the language of your choice. I think that is definitely important, but as of now it's just me.
I have been teaching myself lots of vocab and common phrases and sentences used in my home. In addition, I found a language exchange site where you can learn your language of interest and teach someone else. Books, CDs and homemade flashcards are great, but I enjoy talking to him the most. At this stage I'm having a ball because he's a newborn and he smiles and laughs at everything I say....even if I mess up.