In previous blog posts I explained the Key Strategies 1 and 2 which are about what language you speak to each other.

Do you spend enough time speaking your language with your children?
Even if you always speak your language (OPOL) and get your child to speak to you in your language (1L2U), if they do not hear and speak enough, they will not pick up the basic structures of the language or native pronunciation. Typically, they will sound ‘foreign’ and make more mistakes than they need to.
What level of proficiency are you aiming for?
Some parents want their children to reach native level in both languages, while others might be happy with a much lower level. Up to a certain point, this is a personal choice, but I believe that a native or very near native level of proficiency in the heritage language is a viable goal.
Bilingual speakers of all ages will occasionally transfer words or phrases between languages and will be stronger or dominant in one of them, but this is only a problem if a child is well below native level.
I fully understand why children do not always get enough exposure. As parents, we can feel too stressed, too tired, or simply unmotivated to spend hours every day playing children’s games and doing activities with them! Many times, I had to make a very big effort after a long day at work.
Variations in the dominance of each language
Your main aim is to encourage your child to be a balanced bilingual – a person who speaks and expresses themselves fluently in two languages. However, almost always you will notice that one language is dominant; this is normally the language that your child hears and speaks more of the time.
Changes in your child’s language environment, such as living and studying abroad, making friends with children who speak one of the languages or less contact from a parent, will affect this balance. Sometimes the effect is only temporary. If you spend the summer in your home country, your children may tell you that they were thinking in the home language rather than the previously dominant (perhaps the local) language.
Bilingual speakers also tend to be more proficient and confident in a language depending on what they are talking about, who they speak to and where. If the heritage language is only spoken at home, your child may not have the vocabulary and language skills in subjects they have learned in the dominant language at school (see Chapters 2 and 3 in Part II about strategies to address this situation).
The third Key Strategy increases your child’s abilities in the heritage language, and also nurtures the heritage language identity.
There is research that suggests that the most important time for children when they are learning a language is from birth to around puberty. And of course, it is usually easier to get children to follow your ideas and instructions before they become more rebellious and independent teenagers! You still need to work on language skills at this period, but the teenage years can be unpredictable, as we all know. The more work you have done previously, the easier it will be when they are older.
Think about the number of both passive and active hours of exposure your child needs to reach a truly native level; and remember that you are a mirror for the way your child is going to speak.
For more detailed information and advice, check out the book on Amazon: The 5 Key Strategies of Successful Bilingual Families.