Many parents often ask me how we encourage my son to speak Mandarin on top of speaking Cantonese. Living in the US, both languages are in the minority and we have to make very deliberate choices. I speak fluent Cantonese, and my American-born Chinese (ABC) husband speaks simple Cantonese. We insist on speaking only Cantonese at home, even though I can speak Mandarin as well.
INFANT AGE: Increase exposure as soon as you can.
Playdates and playgroups: Since my son was 6 months old, we have been going to Mandarin singalong classes. At the time of writing my son is/was 3 years old, and we have weekly playdates with native Mandarin speakers. I only speak Mandarin with him during those sessions. He now knows that we speak only Mandarin at these instances so that he is able to acknowledge that it is a different language. We do the same with English.
Repetition: When he started to speak, I would ask him to repeat what I would say in Mandarin; repetition is key!
Ciaohu: Starting from 18 months, we played some CiaoHu in Mandarin to my son. We are strict about screen time, so he only got this during special occasions such as traveling or getting a haircut. (For more about Ciaohu, check out this blog post)
PRESCHOOL AGE and ONWARDS: Immersion, wherever possible.
School: We were lucky to be able to enroll my son in a Mandarin-immersion preschool this year when he turned 3. This has been super helpful and he can speak Mandarin with his teachers without a problem.
Books and Songs: He also plays with reading pen in Mandarin and listen to stories (CD) in Mandarin at home sometimes (I read those books in Cantonese to him first, so he understands the stories before listening to Mandarin). He watches Ocean animals and dinosaurs documentaries in Mandarin on TV. We read a lot at home (I read in Cantonese, and Daddy reads in English). Now I ask him if he wants to listen to stories in Cantonese or Mandarin, but he prefers Cantonese more.
Taking Advantage of Travel: We might be sending him to traditional English-only kindergarten and elementary school, so we plan to do summer camps in Taiwan and Hong Kong in the future to keep up with his Cantonese and Mandarin.
A note to parents who are fluent in Cantonese:
I encourage you only speak 100% Cantonese to your children. Read to them in Cantonese only. Teach them everything you know in Cantonese. If you don't read to them in Cantonese, they won't learn things like science, history, geography, (所謂天文地理, 四字成語等等）Your children will end up knowing a very limited vocabulary, and speak Cantonese like an ABC. Once your children can speak and comprehend Cantonese fluently, then they can learn Mandarin very easily in the future. I didn't learn Mandarin until I was 19, and I can speak business-level Mandarin now.
Good luck!